Flexible Driver Laser for Inertial Fusion Energy

ABSTRACT

Embodiments of a laser system having an extremely large number of small pulsed lasers for irradiating small targets in inertial confinement fusion experiments, high energy density physics experiments, and inertial fusion power plants is more flexible than existing laser systems. Embodiments facilitate finer control of critical features of laser pulses for inertial fusion, as well as significant reduction in development costs and expansion of the community involved in the research relative to existing laser systems. Embodiments produce smooth intensity profiles at the target, large bandwidth that is over two orders of magnitude greater than existing laser systems, and fine control over laser wavelengths, focal properties, temporal pulse shape, and illumination geometry. Properties of each of the small pulsed lasers are individually selectable.

RELATED APPLICATIONS

This application is a continuation application of Patent CooperationTreaty International Application Number PCT/US2012/034289, entitled “AFlexible Driver Laser for Inertial Fusion Energy,” and filed on Apr. 19,2012, which claims priority to U.S. Provisional Patent Application No.61/477,201, entitled “StarDriver—a flexible driver laser for inertialfusion energy,” and filed on Apr. 20, 2011, the priority benefit of bothof which is hereby claimed, and the entirety of both of which is herebyincorporated by reference.

BACKGROUND

1. Field

Embodiments generally relate to the art of inertial confinement fusion,and more particularly to driver lasers for inertial fusion energy andhigh energy density science applications.

2. Related Art

Inertial confinement fusion (ICF) is the science and technology ofachieving controlled thermonuclear fusion wherein matter is compressedand heated to extreme conditions, a state often described by high energydensity physics (HEDP), such that nuclear reactions occur. [Nuckolls1972, Nuckolls 1982]

The primary embodiment of inertial confinement fusion utilizes a verylarge laser system to irradiate a small target containing a small amountof fuel comprised of light nuclei, thereby compressing and heating thefuel to warm density matter conditions. The fuel remains in this stateof high energy density for a very short time, just long enough forsignificant nuclear energy to be released. A related field is inertialfusion energy (IFE), which is the science and technology of usinginertial confinement fusion in a commercial power plant to produceelectricity for commercial use through the national power grid, andother applications. [Campbell 1999a, Campbell 1999b, Hogan 1991,Campbell 1999c, Moses 2009, Nardella 2008, Storm 1991, Dunne 2012,Obenschain 2011]

The US government has supported studies of ICF and IFE for many years.For ICF the goal is to support the nuclear weapons programs at theNational Laboratories, and as such the programs have been administeredby the National Nuclear Security Administration (NNSA). [Campbell 1999c,Paisner 1994, Campbell 1999b, Nardella 2008] The NNSA's mandate isnuclear security—it has no mandate in IFE. The NNSA programs aretherefore specifically limited to nuclear security. As such, a goal ofthe NNSA programs has been to develop the capability to createsignificant neutron fluences, by the most rapid, cost-effective, andreliable path. This path is substantively different from the developmentpath for IFE. Thus, the NNSA programs have not had a focused andextensive effort on the development of the possible optimaltechnological approaches to ICF/IFE in civilian and commercialapplications. The prior art is therefore interesting as background andshows some features of the architecture that can be utilized for IFE,but is silent on some of the essential technologies for IFE/ICF.

The nuclear fusion reaction that has been the primary focus of ICF underU.S. government funding is the nuclear reaction between deuterium (D)and tritium (T) nuclei, which are isotopes of hydrogen. The DT reactioncan efficiently produce a helium nucleus (a) and a neutron (n),releasing significant energy. By collecting the products of thereaction, the released energy can be collected and transformed into heatand electrical power. There are several national security applicationsof ICF including supporting the scientific base for nuclear weapons(Stockpile Stewardship Program), producing materials such as tritium forthe nuclear deterrent, and destroying (transmuting) spent nuclear fuelfrom fission reactors. Both ICF and IFE include the study of othernuclear reactions, and so are not restricted to the DT reaction. Fusioncan also be used in conjunction with fission to produce energy in“hybrid systems.”

In laser-driven inertial confinement fusion (ICF) [Nuckolls 1972,Nuckolls 1982], a large laser irradiates a small target containing anapproximately spherical capsule containing fuel comprised of carefullyselected light nuclei. This approach is called “direct drive.” [McCrory2011]. In an alternative approach called “indirect drive,” the laserenergy is first converted into x-rays in an enclosure (called ahohlraum) containing the fusion capsule [Lindl 1995]. The laserirradiation (or x-rays for indirect drive) ablates the outer surface ofthe capsule. The ablated material has a high momentum, and acts on thecapsule in a manner similar to a rocket engine, forcing the capsulesurface inward. As the capsule implodes in response to the ablationforces, the fuel contained by the capsule is compressed and heated.After the laser pulse has ended, the capsule continues to implode,coasting inward to smaller size, but slowing down. At some point intime, the capsule stagnates briefly. Thereafter, the capsuledisassembles (explodes) under its own pressure and the pressure of thefuel it contains after significant thermonuclear reactions have takenplace. At stagnation, the fuel is ideally in a state of high energydensity, which is defined as having a temperature and density such thatnuclear reactions take place. During the very brief moments ofstagnation (about 10⁻¹⁰ seconds), the fuel pressure reaches values of200 billion atmospheres (higher than that found in the center of manystars), and energy is released by the nuclear reactions in the form ofkinetic energy of the particles produced or created by the nuclearreaction. Some of the released energy is captured in the fuel, heatingthe fuel further, and some of the released energy escapes and iscaptured for subsequent use. In addition to this standard approach ofcompression and heating that occurs through the action of a singledriver laser, other approaches have been investigated for igniting thefuel after it has been compressed. These include shock ignition.[Theobald 2008, Theobald 2009, Perkins 2009] and fast ignition [Tabak1994, Deutsch 1998, Campbell 2006], which are both alternativeapproaches to initiating the burn of DT in high energy density matter.

The time during which nuclear reactions occur, or the time during whichhigh energy density matter conditions are maintained, is essentially thetime elapsed while the fuel and capsule stagnate, before they accelerateoutwards under the action of their own pressure. The fuel and capsuleresist this outward acceleration simply by their own inertia. Thus, itis the inertia of the fuel and capsule that tends to maintain the highenergy density condition achieved at stagnation. Therefore, nuclearenergy is produced only during the time the fuel is inertially confinedat stagnation. Ideally, the stagnation time is long enough to permitmany nuclear reactions to take place and significant usable energy to bereleased. Thus, the art is termed “inertial confinement fusion.”

The art of inertial confinement fusion (ICF) divides generally into twoareas, targets (i.e., fuel and capsule) and drivers, with driver lasersbeing the most common. Both areas involve complex science and technologyfactors, making ICF technically challenging. Experiments to date on ICFhave indicated the general features of the driver laser, and thestructure of the targets, but significant uncertainties remain to beresolved in both areas.

For inertial fusion energy (IFE), an ICF core is embedded in a powerplant. To produce useful amounts of electrical power for the nationalgrid (typically 100 MW to 1 GW), targets must release about 50-100 timesthe laser energy used to drive the implosion, and they must beirradiated by the laser several times a second (˜5 to 15 Hz, dependingon target energy yield). The balance of the power plant deals with thetechnology of capturing the energy released and converting it intoelectricity for the national grid or other uses, the technology oftarget fabrication, and laser operation. The whole enterprise isconstrained by the cost of electrical power (USD/MW-hr) supplied to thenational grid. Power plants currently sell electricity to powerdistributors at a rate between 100 and 150 USD/MW-hr. In IFE, there aremany additional factors to consider beyond the science and technology ofICF. For the laser, these include the need to fire the laser severaltimes a second, the need to limit the down-time of the laser system formaintenance, the need to preserve the quality of the optical pulses usedto irradiate the target, its wallplug efficiency and its capital cost.

As one might imagine, there are many complex scientific andtechnological aspects to ICF and IFE. Some of these aspects have beenidentified, but significant experimentation and technologicaldevelopments will be required before a practical IFE power plant can bedesigned and operated.

A laser system for driving an inertial fusion target must meet astringent set of requirements [Bayramian 2010, Bayramian 2011, Orth1996, Caird 2009]. The requirements include the following: a total laserenergy greater than about 1000 kJ (significant research may be able toreduce this to ˜500 kJ); a small focal spot typically around 500 micronsin size; a wavelength in the visible or ultraviolet region typicallybetween 550 nm and 250 nm; a pulse length of a few tens of nanoseconds;a spatial profile of the total intensity such that on short time scalesof about several picoseconds, the intensity profile in the focus isuniform; a bandwidth adequate to suppress laser-plasma and hydrodynamicinstabilities, typically greater than a few THz; and a complex temporalpulse shape that compresses the DT fuel without excessive heating earlyon, typically beginning with a short spike lasting less than 1nanosecond followed by a smooth rise over many nanoseconds to a peak,followed by an approximately constant power for a few nanoseconds. Forenergy applications, the laser system must include a means of measuringthe position and orientation of a target moving at a high velocity ofapproximately 100 m/s, pointing the laser system at the target, anddelivering the correct laser pulse format to the target. In addition,the laser system must typically do this several times a second. Theserequirements are challenging to meet in one and the same laser system.

The configuration of the driver laser is constrained by available lasertechnology. Numerous research studies over the past 40 years have shownthat target physics requires an illuminating wavelength in theultraviolet (250-350 nm) or perhaps visible (500 nm) ranges. Thesewavelengths are either produced directly as in the output of KrF lasers[Sethian 2002] or by non-linear frequency up-conversion of thefundamental 1053 nm wavelength of Nd:glass lasers to 355 nm. [Paisner1994, McCrory 2012] The wavelength is constrained by the damagethreshold of currently available optics. In the ultraviolet range, todaythe damage threshold of the optics at the exit aperture is typically 1-2GW/cm², or equivalently 3-5 J/cm². To provide a megajoule of ultravioletenergy, given current damage thresholds, the total area of the exitaperture of all the lasers is approximately 100 square meters. There issome advantage to using visible or infrared wavelength, because thedamage threshold is significantly higher than in the ultraviolet at 355nm. There are many other requirements on the laser system for inertialfusion energy production, but here we are concerned mostly with theconfiguration of the laser(s) themselves. Given the total energyrequired, the damage threshold for optics, physics and manufacturingconstraints on laser aperture size, fusion capable ICF and proposedprior IFE laser systems typically have one to several hundredidentically configured laser beams with nominal aperture size of 20-40cm.

The driver laser for the most common approaches to inertial fusiontypically delivers just one pulse to the target that compresses the DTfuel and causes the fuel to ignite and burn. In several advancedapproaches such as the fast igniter [Tabak 1994, Deutsch 1998, Campbell2006], as mentioned above, two laser pulses are envisioned withdifferent functions, one to compress the DT fuel and another to ignitethe fuel. The fuel igniter laser requirements are generally quitedifferent from the fuel compressor laser. Though still uncertain, theigniter requirements include an energy around 100-200 kJ, a pulse lengthapproximately 10 picoseconds, and a focal spot size of tens of microns.In other advanced applications, such as shock ignition [Theobald 2008,Theobald 2009, Perkins 2009], two laser pulses or a single appropriatelyshaped laser pulse may also be used, one to create a low velocitycompression and the second to launch a shock which ignites the fuel.

One of the key requirements for ICF is that the target should maintainits spherical shape while being compressed [Lindl 1995, McCrory 2011,Obenschain 2011]. In direct drive fusion, the laser beams impingedirectly on the target itself, and therefore the spatial profile of thelaser drive should be highly uniform. Drive nonuniformity causes theshape of the target to deform during the implosion by two distinctmechanisms. The first mechanism is simply that if the acceleration ofthe shell is not uniform, the shape of the shell changes as it implodedand so either at stagnation the capsule will not be spherical or thestagnation of the capsule will not be simultaneous around the shell.Then the density and temperature of the fuel may not reach ignitionvalues. The second mechanism is that shell perturbations are amplifiedby the Rayleigh-Taylor and Richtmeyer-Meshkov hydrodynamic instabilitiescausing cold matter from the shell to penetrate the hot fuel atstagnation. This cold matter can also cause the target to fail to reachignition. By and large, effects such as hydrodynamic smoothing tend tomake the acceleration uniformity requirement more significant at longerspatial scales, and the Rayleigh-Taylor and Richtmeyer-Meshkovhydrodynamic instabilities more significant at shorter spatial scales.In general, the overall uniformity requirement is that the rmsnonuniformity should be less than about 0.25% to 1% when integrated overthe e-folding time for the instabilities.

Another requirement for ICF is that the energy of the laser shouldcouple relatively efficiently to the implosion. Laser ablation creates aplasma surrounding the target comprised of the ablated materials. Theplasma surrounding the target can act as a medium in which the laser candrive laser plasma instabilities (LPI) such as filamentation, stimulatedRaman scattering (SRS), stimulated Brillouin scattering (SBS) and the2ω_(pe) instability. LPI scatters the laser drive and in so doingreduces the energy that is coupled to the implosion and redirects thelaser light energy so that it is non-uniform. For example, stimulatedBrillouin scattering causes energy to be transferred from one beam toanother and changes the sphericity of the drive. Also, some LPIprocesses, such as SRS and the 2ω_(pe) instability, can produceenergetic electrons that can “preheat the fuel” and make compressionmore energy intensive. The requirement on the laser drive to avoid LPIis essentially that the laser bandwidth should be at least about 1-2% ofthe laser frequency.

Typically, some type of beam conditioning is required to ensure that theuniformity and bandwidth requirements are met. The laser technologyinvolved is termed “beam smoothing”. There are two existing approachesto beam smoothing: Induced Spatial Incoherence, implemented on Nike[Lehmberg 1998]; and smoothing by spectral dispersion [Skupsky 1989],implemented on NIF, LMJ and Omega, as described below.

As mentioned above, there are several options for the driver laser andthe target design. The two primary target options are indirect drive anddirect drive. In addition, a third approach is a variant of the directdrive approach, known as polar direct drive. The two primary laseroptions in the prior art are the NIF-style Nd:glass laser[Paisner 1994]and the KrF laser[Sethian 2002, Obenschain 2011]. A third butimpractical laser option utilizes a fiber approach[Labaune 2008].Because the highest energy gain (fusion yield/driver energy) is believedto be provided with direct drive target designs, direct drive is todaythe most interesting for IFE. However, the NNSA programs, which aremotivated by national security missions, have invested heavily inNIF-style Nd:glass lasers and indirect drive targets. High gain indirectdrive concepts have also been proposed that may satisfy the requirementsfor IFE. However, the current suite of existing NNSA laser facilities isnot optimally configured for exploring and demonstrating ignition indirect drive targets. In fact, given its national security mission andfocus, a laser concept that meets all the requirements for IFE has notbeen developed in the NNSA program.

At the same time, the reactor technology (the technology of capturing,converting and using the energy released by the targets) has been fundedby a different agency of the US government. The so-called HAPL (HighAverage Power Laser) program at the Naval Research Laboratory inWashington, D.C., LLNL and other participants, has been funded byCongress to develop KrF and diode pumped solid state (DPSSL) lasertechnologies and to study reactor technology and other issues associatedwith IFE such as target production and injection. A great deal ofprogress has been made on both KrF laser technology [Sethian 2002], onadvanced solid state laser approaches[Bayramian 2007], and on someelements of the reactor. However, these studies have been directed tounderstanding the balance of elements in an IFE power plant and thedesign of the reactor which captures the energy released from thetargets. They have not been directed to understanding how to configurethe laser driver so that the full set of target requirements can be metsimultaneously.

Under the NNSA program, studies of ICF have been carried out at theLawrence Livermore National Laboratory (LLNL) [Haynam 2007] [Paisner1994] [Campbell 1999], the Laboratory for Laser Energetics (LLE) at theUniversity of Rochester, N.Y., [LLE 2012] and the Naval researchLaboratory in Washington D.C. [Sethian'2002]. The earliest studies datefrom the early 1970's. The lasers used at LLNL and LLE are generallyreferred to as NIF-style lasers, after the National Ignition Facility atLLNL.

The NIF-style laser [Haynam 2007] is a flash-lamp pumped Nd:glass laser.Several generations of glass lasers have been built at LLNL, each onesignificantly larger than the previous laser. The configuration of thislaser is a master-oscillator/power amplifier, in which a small laserbeam with the desired pulse shape and spatial mode properties is firstgenerated in the master-oscillator and then amplified in a series ofpower amplifiers and transport optics before being focused onto atarget[Paisner 1994]. The more recent Nd:glass lasers have included afrequency conversion device that converts the wavelength of the laserbeam from 1064 nm in the infrared to 355 nm in the ultraviolet, beforethe wavelength-converted beam is focused onto the target, as well asother “beam conditioning systems such as phase plates to control thefocal spot profile [Paisner 1994, Haynam 2007]. This laser approach ismature (for example, NIF is the sixth laser build by LLNL since theearly 1970's) and so has minimal risk as a choice for a driver laser forICF. While low risk may be an attractive feature for nuclear securityprograms, when NIF is measured against the requirements for IFE, severalshort-comings are evident. For example, its efficiency (optical energyout/electrical energy in) is less than 1%, whereas the requirement isaround 10%. It has a very narrow bandwidth, about 0.25 THz, whereas therequirement to suppress LPI is believed to be around 1-2% of the laserfrequency, or perhaps greater. The smoothing technique used by NIF atLLNL and Omega[LLE 2012] is smoothing by spectral dispersion(SSD)[Skupsky 1989, Skupsky 1993], which is essentially a means ofcausing the laser beam to shimmer at a high rate at the target. Thesmoothness of the LLNL laser pulses at the target is significantlyhigher than 20%, and the smoothness of the LLE laser pulses is about10%, to be compared with the requirement which is typically 0.25%. It isdifficult if not completely impractical to configure a NIF-style laserto deliver more than about 1 pulse per second, without significantadvances in laser amplifier technology, whereas the requirement for IFEis about 5-15 pulses per second. Achieving a high repetition rate aNIF-style laser is challenging because of the large beam aperture. Alsothe wallplug efficiency of a NIF-style laser is ˜1% today. Again withsignificant advances in laser architecture and amplifier design this canbe improved, but it is unlikely to meet the IFE requirement.

The NIF-style laser configurations used by LLNL have included severalseparate beam lines, all nominally identical and delivering the samepulses to the target. The latest system at LLNL, the National IgnitionFacility [Paisner 1994][Campbell 1999a][Haynam'2007] uses 192 beams in48 clusters delivering pulses to a target chamber about 5 meters inradius, containing a target placed at its center. The final focusinglenses are about 7 meters from the target. The total energy per pulsedelivered by NIF is up to 1.8 MJ at 355 nm, delivered in a pulse lengthof approximately 25 ns. The latest system at LLE is Omega [McCrory 2012]which uses 60 beams and delivers about 30 kJ of energy in theultraviolet in about 1 nanosecond.

Variants of the NIF-style laser adapted to some of the specialrequirements of commercial energy production have been studied [Caird2009, Storm 1991, Campbell 1999a, Hogan 1991], and some initialexperiments have been carried out to test them [Bayramian 2007].

The laser used at NRL is Nike, a Krypton Fluoride (KrF) laser. [Sethian2002] The KrF laser operates in the ultraviolet at 248 nm. It also isconfigured as a master-oscillator power amplifier. The KrF laser has thebest smoothness achieved to date, which is less than 1%. The KrF laseruses the smoothing technique of induced spatial incoherence [Lehmberg1993, Lehmberg 1998, Lehmberg 2000, Lehmberg 2005]. The Nike laserbandwidth is less than 5 THz, and its efficiency can potentially be ashigh as 7%. The optical configuration of the KrF laser involves passinga few short pulse beams in sequence through an amplifier and thenpassing each beam through an optical delay so the beams arrive at thetarget simultaneously. The KrF laser has some attractive features forIFE[Obenschain 2011], compared to the NIF-style laser, a flexible KrFdesign that meets all the requirements has proved elusive.

A regular feature of the prior driver lasers is that their configurationis typically master-oscillator/power-amplifer (MOPA), where a primaryoscillator provides a single small seed pulse that is optically dividedusing beam-splitters into many seeds. All seed pulses are thereforecoherent with each other. Individual seed pulses may be shaped andmodulated in slightly different ways. Each seed pulse passes through aseparate amplifier or a system of amplifiers, one for each seed, wherethe seed pulse is amplified and subsequently focused onto the target.Thus the pulses from all the beams focused onto the target arecoherently related. For the NIF-style laser, the primary seed is highlyspatially coherent. The modulation is used in conjunction withdiffraction gratings according to the SSD methodology. The opticalphases of the beams across each beam aperture are therefore highlycoherent. Even after passing through a phase plate, a high degree ofspatial coherence remains. Thus every time the laser fires, the samephase relationships exist between the beams impinging on the targetwhich compromises the smoothness of the drive at the target. Indeed, itis challenging for a Nd:glass laser configured as in the prior art todeliver a laser drive at the target with the required smoothness fordirect drive target.

For the KrF laser system, the primary seed is multimode, and after beingsplit, it is amplified and imaged onto to the target. The asymptoticsmoothness of the drive at the target is limited by the spatialintensity profile at the target associated with each spatial mode, andthe rate of smoothing is controlled by the bandwidth. While the KrFlaser has achieved the best asymptotic smoothness in the prior art, thesmoothing rate is limited by the (gain-narrowed) bandwidth of the KrFlaser amplifiers, which is typically 1-2 THz.

Other lasers for ICF are planned. The Laser MegaJoule in France[Bettinger 1999] was developed in close scientific collaboration withLLNL. It is not yet completed, but as currently planned it will have thesame smoothness, bandwidth and mean wavelength as the NIF-style laser atLLNL. It does not represent a substantively different approach to ICFand IFE from NIF. NIF-style lasers are also being considered in Europe(e.g., HiPer) [Dunne 2007] and Japan (Firex) [Azechi 2006], Russia(Unnamed) [Dean 2012] and China (Divine Light 4) [Dean 2012]. All thesesystems have the same general features as NIF in regard to smoothness,bandwidth, and mean wavelength. Consequently, none of them meet all therequirements for direct drive ICF or IFE. These laser systems all have afew, large aperture (˜40 cm) beams, and face challenges of adequateflexibility in pulse shaping, frequency conversion, bandwidth, beamsmoothness, and smoothing rate. The recent proposal for an IFEdemonstration known as LIFE (Laser Inertial Fusion Energy) [Bayramiam2009, Bayramian 2010, Caird 2009, Moses 2009] is an adaptation of theNIF-style laser to IFE, and so it too will have significant challengesto meet all of the IFE requirements.

A different approach has been proposed by an international group ofscientists where the laser system is built from a very large number(more than 10,000,000), very small individual single-mode Ytterbiumfiber lasers [Labaune 2008]. Each fiber laser output is collected by alens and focused on the target. To focus on the target, the lensdiameter has a minimum size, and this limits the number of lenses. Thelens size is such that the entire 4 π solid angle surrounding the targetwould be significantly filled by approximately 10-20 million lenses. Thenumber of fiber lasers must obviously be significantly less than this.To deliver one megajoule of light, each laser must deliver significantlymore than 100 mJ. This is well in excess of the state of the art insingle mode fiber laser technology, which is about 10 mJ. Moreover,there are optical engineering challenges such as beam pointing that areextreme in this approach. Even though the fiber approach has beendescribed in the literature related to IFE, it could not be used for IFEwithout significant invention and development in both large mode areafiber lasers, and precision optical alignment.

All of the laser systems either utilized or conceived in the prior artface significant challenges in meeting simultaneously all therequirements for a successful implosion of a high gain target for ICF orIFE, and all of them have limited flexibility to accommodate differenttarget designs.

SUMMARY

Embodiments include a configuration of a large laser system for use ininertial confinement fusion and specifically tailored for inertialfusion energy production and HEDP. Embodiments include configuring adriver laser as a very large number of small beamlets, which areindependently configurable. Embodiments provide extreme flexibility inobtaining the required laser drive at the target, through control of thefrequencies and other properties of individual beamlets. The beamsmoothing and bandwidth of the laser system enables the laser system tomeet all of the requirements on the total laser drive, including,wavelength(s), bandwidth, smoothness, temporal pulse shape, and focusingof the laser drive. It also enables time sequencing of the pulses fromindividual beamlets or groups of beamlets, so that all of the featuresof a laser may be finely controlled at each instant during the entirelaser drive pulse. Embodiments also facilitate significant cost savingsby sharing support hardware among many beamlets, and reduced developmentcosts for a single beamlet. Embodiments also enable greater wall-plugefficiency for the driver laser, minimizing the size and cost of thelaser hardware itself.

In an embodiment, a laser system includes a plurality of pulsed lasersthat emit laser pulses. The plurality of pulsed lasers are configuredsuch that all of the plurality of pulsed lasers emit a laser pulse thatirradiates a target within a same time window of less than about 100 ns.At least two of the pulsed lasers have different central opticalfrequencies such that the central optical frequencies of theirrespective emitted laser pulses differ by more than 1 THz.

The plurality of pulsed lasers may be configured such that each pulsedlaser emits a laser pulse that irradiates the target within the sametime window such that the target releases thermonuclear energy inresponse to the irradiation.

The laser system may further include a laser controller that controlsthe pulsed lasers such that each pulsed laser irradiates the target witha laser pulse substantially simultaneously with the other pulsed lasers.

The plurality of pulsed lasers may include at least 512 and less than262,145 pulsed lasers.

The plurality of pulsed lasers may be configured to deliver the laserpulses to the target in a substantially spherical distribution.

The plurality of pulsed lasers may each include an exit aperture and theplurality of exit apertures may be distributed substantially sphericallyaround the target.

A distribution of the central optical frequencies of the pulsed lasersmay be correlated with a direction of propagation and a focal spotlocation of the respective laser pulses emitted by the pulsed laserstoward the target according to a predetermined prescription.

The predetermined prescription may substantially maximize a spatialuniformity of intensity of the plurality of laser pulses at a surface ofthe target as computed from a ratio of the root mean square variation inthe intensity over the surface of the target to the average value of theintensity over a time interval during which the plurality of laserpulses irradiate the target surface.

According to a predetermined prescription, a variation in spatialuniformity of intensity of the plurality of laser pulses at a surface ofthe target as computed from a ratio of the root mean square variation inthe intensity over the surface of the target to the average value of theintensity over a time interval during which the plurality of laserpulses irradiate the target surface may be less than about 0.25%.

A smoothing rate of the summation of the plurality of laser pulses ofthe laser system at the target may be substantially maximized at aspatial scale length of between about 10 and about 100 microns,according to the predetermined prescription.

The plurality of laser pulses from the laser system that irradiate thetarget may be substantially smoothed at a rate faster than about 30 THz,according to the predetermined prescription.

A central optical wavelength of each pulsed laser may be between about250 nm and 2500 nm, and the root mean square bandwidth of the lasersystem may be greater than about 1 THz.

Temporal pulse shapes of the at least two of the pulsed lasers havingdifferent central optical frequencies may be substantially differentfrom each other.

Optical states of polarization of at least two laser pulses thatirradiate the target from different respective pulsed lasers may besubstantially different.

A temporal pulse width of at least one of the plurality of laser pulsesmay be less than about 50 ps.

A first temporal pulse width of a first laser pulse of the plurality oflaser pulses may be between about 1 ns and 100 ns, and a second temporalpulse width of a second laser pulse of the plurality of laser pulses maybe less than about 50 ps.

At least two of the plurality of laser pulses may irradiate the targetsurface at substantially different times.

An angle between propagation directions of any two laser pulses fromrespective pulsed lasers whose central optical frequencies differ byless than about 250 THz may be greater than about 0.01 radians.

In another embodiment, a laser system includes a plurality of at least512 and less than 262,145 pulsed lasers that emit laser pulses toward atarget. At least two of the pulsed lasers have different central opticalfrequencies such that the central optical frequencies of theirrespective emitted laser pulses differ by more than about 1 THz. Aplurality of exit apertures are spatially distributed around the targetsuch that the laser pulses from each of the plurality of pulsed laserspass through a separate one of the plurality of exit apertures toirradiate the target from a different direction. The laser system alsoincludes a laser controller that controls the plurality of pulsed laserssuch that all of the plurality of pulsed lasers irradiate the targetwith a laser pulse within a same time window of less than about 100 ns.

At least two of the pulsed lasers may have different central opticalfrequencies such that the central optical frequencies of theirrespective emitted laser pulses differ by more than about 2 THz.

A distribution of the central optical frequencies of the pulsed lasersmay be correlated with a direction of propagation and a focal spotlocation of the respective laser pulses emitted by the pulsed laserstoward the target according to a predetermined prescription.

The predetermined prescription may substantially maximize a spatialuniformity of intensity of the plurality of laser pulses at a surface ofthe target as computed from a ratio of the root mean square variation inthe intensity over the surface of the target to the average value of theintensity over a time interval during which the plurality of laserpulses irradiate the target surface.

According to the predetermined prescription, a variation in spatialuniformity of intensity of the plurality of laser pulses at a surface ofthe target as computed from a ratio of the root mean square variation inthe intensity over the surface of the target to the average value of theintensity over a time interval during which the plurality of laserpulses irradiate the target surface may be less than about 0.25%.

In another embodiment, a method of driving an inertial confinementfusion reaction for inertial fusion energy generation includes emittinga plurality of laser pulses from a plurality of pulsed lasers. Centraloptical frequencies of at least two of the pulsed lasers are differentfrom each other by more than about 1 THz. The method also includesdirecting the plurality of laser pulses toward a target from differentexit apertures along different propagation directions such that all ofthe plurality of laser pulses irradiate different portions of the targetwithin a same time window of less than about 100 ns.

Central optical frequencies of at least two of the pulsed lasers may bedifferent from each other by more than about 2 THz.

The method may further include correlating a distribution of the centraloptical frequencies of the pulsed lasers with a direction of propagationand a focal spot location of the respective laser pulses emitted by thepulsed lasers toward the target according to a predeterminedprescription.

According to the predetermined prescription, a variation in spatialuniformity of intensity of the plurality of laser pulses at a surface ofthe target as computed from a ratio of the root mean square variation inthe intensity over the surface of the target to the average value of theintensity over a time interval during which the plurality of laserpulses irradiate the target surface may be less than about 0.25%.

The method may further include correlating a distribution of opticalpulse shapes of the pulsed lasers with the distribution of propagationand focal spot location of the respective laser pulses emitted by thepulsed lasers toward the target according to the predeterminedprescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the minimum number of beamlets required for beamlets ofvarious apertures, according to an embodiment.

FIG. 2 illustrates elements of a driver laser and reactor for an IFEpower plant and the general path followed by a pulsed laser beam in theIFE power plant, according to an embodiment.

FIG. 3 illustrates a spherical arrangement of beamlets around a targetchamber, according to an embodiment.

FIG. 4 illustrates a conical arrangement of beamlets around a targetchamber, where the beamlets are grouped into two clusters, according toan embodiment.

FIG. 5 illustrates details of the relationship between a beamlet and thetarget chamber wall, according to an embodiment.

FIG. 6 illustrates a block diagram of a beamlet of a driver laser forinertial fusion energy, according to an embodiment.

FIG. 7 illustrates a schematic diagram of a beamlet for a driver laserfor inertial fusion energy, according to an embodiment.

FIG. 8 illustrates a total power of all beamlets in a driver laser as afunction of time, according to an embodiment.

FIG. 9 illustrates smoothness of the laser drive at the target as afunction of time, according to an embodiment.

FIG. 10 illustrates the smoothness of the laser drive at the target as afunction of time, comparing prior driver lasers with an embodiment asdescribed herein.

FIG. 11 illustrates an optimal distribution of four frequencies amongthe beamlets, according to an embodiment.

FIG. 12 illustrates an optimal distribution of five frequencies amongthe beamlets, according to an embodiment.

FIG. 13 illustrates an optimal distribution of nine frequencies amongthe beamlets, according to an embodiment.

FIG. 14 illustrates a spatial spectrum of a group of beamlets, accordingto an embodiment.

FIG. 15 illustrates an exemplary pattern of spatial frequency spectrumof four frequencies.

FIG. 16 illustrates an exemplary pattern of spatial frequency spectrumof nine frequencies.

FIG. 17 illustrates an exemplary pattern of spatial frequency spectrumof sixteen frequencies.

FIG. 18 illustrates a controller of the driver laser, according to anembodiment.

FIG. 19 illustrates a method of driving an inertial confinement fusionreaction for inertial fusion energy generation, according to anembodiment.

DETAILED DESCRIPTION

Current lasers used for ICF studies present multiple challenges to meetall the beam smoothing requirements resulting in reduced energyavailable on target, system complexity, or compromised beam smoothingperformance. Therefore, to address these system deficiencies, themultiple aperture laser as herein described is a new approach to meetboth the bandwidth and uniformity requirements. In the multiple apertureapproach, many small lasers with a small aperture of a few centimeters,and numbering as many as 100,000 or more, are directed simultaneously tothe target. Each of these many small lasers is referred to herein as a“beamlet.” In various embodiments, the beamlets are not all identical,but have a wide variety of beam features. For example, the beamlets maydiffer in their distribution of frequency among the beamlets and thetotal bandwidth of the ensemble of beamlets. Moreover, each beamlet mayhave a small aperture, for example several centimeters.

Each beamlet may have a spot size at the target that is smaller than thedimensions of the target itself. As efficient energy coupling generallyrequires matching the size of the laser drive to the size of the target,in various embodiments, the efficient energy coupling requirement may bemet by the summation of all the spots from all the beamlets at thetarget. The spatial structure in the laser drive at the target may bedetermined by both the interference of all the beams and thearchitecture and methods for ensuring that the size of the laser profileat the target matches the target dimension.

From the point of view of target physics, the low L-mode portion of thespatial mode spectrum includes the spatial wavelengths that are mostsignificant for acceleration uniformity, whereas in the high L-modeportion of the spectrum, hydrodynamic instabilities are moresignificant. From the point of view of laser technology, however, thelong-wavelength portion of the spectrum deals with the need to match thesize of the laser drive to the target dimension, whereas the shortwavelength portion deals with the interference patterns betweenoverlapping beams. The appropriate division of the spectrum is generallydifferent for target physics than for laser technology. Here it naturalto adopt the laser technology point of view.

Optical techniques for achieving low L-mode uniformity can potentiallycompromise the high L-mode uniformity. For example, low L-modeuniformity may be attempted by tiling the target disc with many spots,i.e. pointing individual beamlets in slightly different directions sothat their diffraction-limited spots do not necessarily overlap.However, as the number of beamlets that overlap in the target disc isreduced, the high L-mode uniformity deteriorates. If the beamlets tilethe target disc with many distinct tiles, the number of overlappingspots is smaller. Then, the high-spatial frequency uniformity can becomeunacceptably large. Therefore, the method for obtaining low L-modeuniformity should not reduce the number of overlapping beamletssignificantly.

The challenge of configuring a laser system to meet all the targetrequirements simultaneously has not been previously addressed. The lasersystems that have been proposed and the laser systems that have beenbuilt meet some of the requirements, but not all. However, in variousembodiments, embodiments of a driver laser as described herein addresssimultaneously meeting all the requirements. Various embodiments of adriver laser as described herein have a large number of beams and offera practical, cost-effective laser system that simultaneously meets allof the target requirements, in sharp contrast to the current lasersystems. If the beamlets are differentiated according to theprescriptions set forth herein, the laser drive according to variousembodiments will have the flexibility to meet all of the requirements.Beamlets may be differentiated from each other in many ways, includingtheir wavelengths, temporal pulse shapes, spot sizes on target, spatialprofiles on target, laser pointing, and polarization.

Embodiments employ a multiple aperture approach to IFE laser systems, inwhich many (N in total number) individual laser systems (“beamlets”) aredirected independently to the target area. A typical IFE configurationplaces all N laser beam apertures closely together so that the beams alllie within a cone of small angle when viewed from the target. The beamsmay also be distributed around the target, in which case they maysubtend a few percent of the 4π solid angle when viewed from the target.In order to control costs, the lasers may have a minimal number ofcomponents, they may have many common components, and they may sharesupport facilities such as pulsed power and control hardware. To someextent, then, the multiple aperture laser system can be viewed as asingle laser/electro-optic system with many independently directedoutput apertures. Each individual laser system may include anoscillator, amplifiers, and transport optics such as spatial filters.Each individual laser may be presumed to have a minimum number ofcomponents, with no output beam conditioning, phase plates, or adaptiveoptics. The lasers may be presumed to have a nominal wavelength in theinfrared, so there may be a frequency convertor between the output ofthe laser and the final focusing optics. The wavelengths of the lasersneed not be the same. In fact, if all the beamlet lasers use the samegain medium, the total bandwidth may be too small to affect targetperformance. For ICF and IFE, bandwidth is important, so in embodiments,the wavelength of each beamlet in the multiple aperture laser may beindependently specified. However, we recognize that in the extreme caseof N different wavelengths, some of the cost reduction coming fromhaving shared components may be lost.

The oscillator of each laser may be a resonator containing a gainmedium. Even though the source of light from the oscillator may beamplified spontaneous emission (ASE), the resonator may provide modediscrimination such that all modes but one of the ASE from the gainmedium experience a high loss. The oscillator may also include beamspatial profile conditioning to optimize the laser performance and thecoupling of the laser beam to the target. This may result in an outputfield from the oscillator that is preferably a single frequency(longitudinal) mode with a specific, appropriately chosen spatialprofile. At a reference time t₀, the input to each laser chain may berepresented as shown in Eq. 1:

E(x,y,t ₀)=e ^(iξ·E) _(in)(x,y,t ₀)  (Eq. 1)

The field E_(in) describes the temporal envelope of the wave packetproduced by the oscillator. The phase ξ is determined by the quantumsource of the emission in the oscillator. On any pulse from theoscillator, the phase ξ is a constant in time, but over an ensemble ofmany pulses, the phase ξ is uniformly distributed from 0 to 2π. The beamin the target area is the sum of the beams of a single (amplified) pulsefrom each oscillator, and is described by a set of quantum phases{ξ₁:1≦i≦N}. This set of phases is fixed for each target shot, but it isa different set for each target shot. We may not average over this setof phases in calculating the intensity profile on any given shot, and weshould be mindful of the range of possibilities on any one shot derivedfrom the range of values these phases may take.

The pulse from each oscillator at time t in a particular target shot maybe expressed as Eq. 2:

E(x,y,t)=e ^(iξ) ·E _(in)(x,y,t)e ^(iω(t−t) ⁰ ⁾  (Eq. 2)

This beam propagates to the output of the laser, being amplified anddistorted by the gain profile and aberrations in the laser optics. Thebeam just before reaching the frequency convertor may be expressed asEq. 3:

E(x,y,t)=e ^(iξ+iωt) ∫dx′dy′dt′e ^(iωR/c) D(x,y;x′,y′,t _(ret))E(x′,y′,t_(ret))  (Eq. 3)

where R is the optical distance from (x′,y′) to (x,y). The propagator D(or green's function) representing the optics and gain of the lasersystem takes into account beam magnification, the gain profile, andoptics aberrations. The propagator D also accounts for the optical groupdelay t_(d) between the oscillator and the frequency convertor. Theoptical group delay t_(d) may be expressed as Eq. 4:

$\begin{matrix}{t_{d} = {\sum\limits_{{laser}\mspace{14mu} {compnents}}\; {s_{i}/v_{gi}}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

Here s_(i) is the propagation distance in the i-th laser component andv_(gi)−[dω/dk]_(i) is the phase velocity of the beam in that component.The phase of the beam at the frequency convertor may therefore beexpressed as arg(E_(in))+ωR/c. After frequency conversion, the beamacquires a phase that depends on the frequency conversion efficiency anda phase representing the propagation of the beam to the target area. Thenet of these effects is that the phase of the beam in the target area isξ+ω(R+f)/c+ψ_(fc). Controlling this phase may require controlling all ofthese phases in each beam. Controlling the phases of the time delays andfrequency conversion to a fraction of an optical cycle may beprohibitively expensive and technologically challenging. Moreover,without such control, there may be no point in attempting to control thequantum phase ξ. Therefore, the multiple aperture laser may not controlthe relative phases of the beams in the target area.

The beam at the frequency convertor typically has both phase andamplitude variations. In order to optimize the conversion efficiency, itis usually a design requirement that the phase and amplitude variationsof the beam at the frequency convertor be controlled. The conversionefficiency is quite sensitive to phase aberrations, yet relativelytolerant of amplitude variations. A diffraction-limited beam at thefrequency convertor may be presumed to have a flat phase by design, anda smooth amplitude profile. For conversion from the infrared at 1ω tothe ultraviolet at 3ω, the variation in local beam direction, λ|gradφ|may be limited to (Δθ_(x),Δθ_(y)) in the x- and y-directions, which arenumerically a small fraction of the angular acceptance of the crystal inthese directions. For tripling 1053 nm light to 355 nm using KDP,typical values for (Δθ_(x),Δθ_(y)) are in the neighborhood of 100microrad. These limits can be compared with the maximum angularvariation that can be tolerated if the beam is to be smaller than thetarget itself. The angle subtended by the target is r/f, where r is thetarget radius and f is the focal length of the final focusing lens. Thisangle is typically about 100 microrad. Therefore, the target size may bethe controlling factor in phase variation, rather than frequencyconversion. The variation in the beam amplitude tolerable by thefrequency convertor can be up to about a factor of 2, so long as thespatial wavelength of the amplitude variations is such that diffractioncan be neglected as the beam traverses the crystals. Thus, the smallestspatial wavelength for amplitude variation that can be tolerated by thefrequency convertor is typically in the neighborhood of √/Lλ, where L isthe crystal thickness, or about 100 microns, and the angle associatedwith this is about 10 mrad. Therefore, to a good approximation, if thebeam is to be smaller than the target, the frequency convertorefficiency will not be compromised.

Under these circumstances, and if the 1ω beam at the frequency convertoris relatively uniform in spatial profile, the conversion process can bedescribed by a relatively simple model, where the phase of the 3ω beammay given by Eq. 5:

φ_(3ω)(x,y,t)=3φ_(1ω)(x,y,t)  (Eq. 5)

and its amplitude may be given by Eq. 6:

A _(3ω)(x,y,t)∝[A _(1ω)(x,y,t)]³  (Eq. 6)

The optical frequency ω, spatial frequency bandwidth Δκ, and thetemporal bandwidth Δω at 3ω) are all three times as large as those ofthe input 1ω beam. (The spatial scale lengths in the 3ω beam tend to bethree times smaller than those in the 1ω beam.) Thus, the conversionprocess may preserve the overall beam divergence Δκ/k=M′λ/D, and thefractional frequency bandwidth Δω/ω. Note that M for the 3ω beam isthree times that of the 1ω beam, so M′λ is conserved by frequencyconversion. Limitations on Δκ/k or M′λ that are derived from the need tooptimize the coupling to the target at 3ω can therefore be calculatedfor the 1ω beam directly using these scaling laws, without explicitlycalculating the influence of frequency conversion on the beam spatialand temporal profiles.

Referring now exclusively to the 1ω beam, if the beam is circular anduniform, the spot profiled may be expressed as [(2/u)J₁(u)]² where J₁ isBessel function, whose first zero lies at radius 1.22λf/D. This profilecontains significant energy outside the first zero. If the 1ω beam isrectangular and uniform, the spot has a sinc² profile with zeros atx=±λf/D. This sinc² profile also has significant energy outside itsfirst zeros. If the complex amplitude is not uniform, the spot profilemay become ragged, and its RMS size may increase, for example to M λf/D,where M is defined by this equation. Ideally, from the perspectives ofboth cost and performance, all the energy of the 3ω beam will couple tothe target. One way to accomplish this is to arrange for the spot sizeof an individual laser beam to underfill the target cross-sectionsignificantly, making up the required spatial profile of the drive bycareful pointing of many undersized spots. This in turn implies that atany point in the focal plane, the N beams are not completelyoverlapping, so that the effective number of beams that contribute tobeam smoothing is somewhat less than N. If the beams under-fill thetarget cross-section by a factor of 2, so that their spot radius isdecreased by √2, then the effective number of overlapped beams is alsoreduced by about a factor of 2, causing an increase in the long-time orasymptotic intensity variation in the focal profile by √2. If the beamsunder-fill the target cross-section by a factor of 9, the asymptoticintensity variation increases by a factor of approximately 3. Thus,there is a trade-off between efficient coupling to the target andasymptotic beam smoothness. Moreover, although an analysis of thetrade-off between target coupling and asymptotic smoothness has not beendone for IFE targets, an acceptable effective asymptotic smoothness isknown to be about 0.25%, and this is eminently compatible with efficientcoupling. The effective number of overlapping beams depends on the spotsize of each beamlet. Partly overlapping beamlets have importantconsequences for the statistics of the beam, and we shall return to thistopic below.

The spatial profile of a beamlet at the target is related by diffractionto its spatial profile at the amplifiers, and the extraction efficiencyof the amplifer is related to the spatial profile of the beam there. Theoverall system efficiency for an IFE system is a tradeoff between thetarget coupling and gain and the extraction efficiency of theamplifiers. The optimal spatial profile at the target has a centralpeak, falls slowly away from the center, and has minimal energy at radiioutside of the target radius. The optimal spatial profile will bedetermined by the well-known principles of diffraction, amplifierextraction and target coupling [Lehmberg 2000], and will depend to someextent on the individual beamlet aperture and the target design. It maybe determined by the application of these principles to any embodiment.A very effective profile is a cos² θ profile, but the sinc² profile andsupergaussian profiles are both close to optimal.

This situation contrasts with current ICF driver lasers. For Omega [LLE2012] and NIF [Haynam 2007], the spatial profile on target is controlledby phase plates which are designed to provide a spatially smoothasymptotic profile with steep edges at the target. Such phase platestypically have large amplitude features over the full aperture whilemaintaining otherwise diffraction-limited performance. They arechallenging and expensive to design and fabricate. For Nike[Sethian'2002], the target profile is controlled by adjusting theprofile of the ASE source and imaging this profile to the final focalplane. The amplifiers are typically in the far field of the source, sothe beam profile in the amplifiers may be roughly uniform. Inembodiments of the multiple aperture approach, it is desirable tominimize the complexity of the lasers, and in a small aperture laserthere may be a few spatial modes and further spatial tailoring may notbe required. It may be challenging (or even impossible) to tailor thespatial profile of the individual focal spots without also tailoring thespatial profile of the beam in the amplifiers. However, with a largenumber of beams, such tailoring is greatly simplified and may even beunnecessary. The spot radius of an individual laser, M′λ f/D, must beless than the target radius, so the aperture diameter D may e determinedby Eq. 7:

$\begin{matrix}{D \geq D_{0} \equiv \frac{M^{\prime}f\; \lambda}{2r}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

The relevant target radius may be the initial radius, or the radius atignition, which is smaller. Using the initial target radius r=0.5 mm,f=10 m, and λ=⅓ μm, D is about 10 mm for a uniform beam profile. Thefluence of the beam at the 3ω optics specifies the total beam area at3ω. If the energy delivered to the target is E, and damage threshold ofthe optics at 3ω is J_(d), the number of beams must satisfy Eq. 8:

$\begin{matrix}{{N \geq \frac{E}{\pi \; D^{2}J_{d}\eta_{T}}},} & \left( {{Eq}.\mspace{14mu} 8} \right)\end{matrix}$

for circular spots or Eq. 9:

$\begin{matrix}{N \geq \frac{E}{D_{x}D_{y}J_{d}\eta_{T}}} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$

for rectangular spots, where η_(T) is the coupling efficiency of thelaser output energy at 3ω to the target. The minimum number of beamletsand beamlet aperture is plotted in FIG. 1. The minimum number ofbeamlets decreases as the beamlet aperture is increased. The maximumnumber of beamlets should be selected so that the beamlets irradiate thetarget efficiently without the need for coherent addition. The overallsystem efficiency has a broad optimum centered on a beamlet aperture of5 cm. Also shown in FIG. 1 is an inset of a portion of the data plottedin FIG. 10, showing the beam smoothing achievable with a multipleaperture laser system.

The cost of the laser system scales with both the number of beams andthe beam aperture. One model of the cost scaling is as follows in Eq.10:

$\begin{matrix}{C = {{C_{ref}\left( \frac{N}{N_{ref}} \right)}^{p_{N}}\left( \frac{D + D_{m}}{D_{ref} + D_{m}} \right)^{2_{p_{D}}}}} & \left( {{Eq}.\mspace{14mu} 10} \right)\end{matrix}$

where the subscript “ref” denotes a reference design point. D_(m)represents the fact that even a laser of vanishing aperture has costsassociated with the hardware. In this model, the cost scales as a powerof N and a power of (D+D_(m)). The constant C_(ref) refers to the costof the reference design, using the future technologies that will beavailable at the time in the future when the laser is constructed. Whileit is highly speculative to build a cost model without a laser designand in the absence of insight into future innovations and manufacturingtechnologies, some comments can be made. First, the high level ofmodularity in the multi-aperture concept and the common features withdiode and semiconductor technology suggest that the constant C_(ref)will be significantly smaller for embodiments of the multi-aperturelaser than for existing laser concepts. Secondly, as one varies thenumber of beams and the beam apertures within this future technology,one may possibly estimate the scaling law exponents from relatedtechnologies as in the range of about 0.5 to 0.7. Note that ND_(x)D_(y)is fixed by the 3ω damage limit of the optics. So if the fluence of thebeams at the 3ω optics is held constant, N and D scale oppositely.Consequently the (future) cost may not vary significantly as the numberof beams is varied. Thirdly, the costs of diode lasers have beendropping rapidly as the diode laser industry develops. It is notunreasonable at this time to project diode costs in the range of a fewcents/Watt in a fusion power economy. This suggests a cost model where(a) the most significant (future) cost savings are obtained by usingadvanced laser technologies, especially future diode production costs,and (b) that the (future) cost depends mostly on the damage threshold ofthe 3ω optics and varies little with the number and apertures of thelaser beams. Both these statements about future costs are suggestive atbest in the absence of a specific laser technology and laser design inhand.

The flexibility in configuring the beams around the target isdemonstrated by noting that if the N beams are gathered into a tightlypacked cone when viewed from the target, the cone angle α is small, asgiven in Eq. 11 below:

$\begin{matrix}{{\alpha \approx {\frac{1}{f}\sqrt{\frac{E}{\pi \; J_{d}}}}} = {\frac{D}{f}\sqrt{N}}} & \left( {{Eq}.\mspace{14mu} 11} \right)\end{matrix}$

For E=0.5 MJ for two-sided illumination, J_(d)=4 J/cm², and f=10 m, theminimum cone angle is approximately 0.22 radians ˜13 degrees. Anyconfiguration between this tight cone to uniform distribution over 4πsteradians may be selected.

In various embodiments, a large number of lasers may be configured in anappropriate angular distribution so that their output beams are directedto a small target volume, typically about 1 mm³, such that the distancebetween the final optical element of each beamlet is several metersaway. The distance from the exit aperture of each beamlet to the targetvolume is determined by three main requirements. First, the total solidangle subtended by all the beamlets at the target must be significantlyless than 4π steradians. Second, the total area of all the beamlets mustbe larger than E/J_(d), where E is the total laser energy delivered tothe target volume on each pulse and J_(d) is the threshold for opticaldamage of the last few optical elements comprising a beamlet. Third, theexit apertures of the beamlets can be at any appropriate locationsthroughout the target chamber, consistent with the minimum cone anglediscussed above, and the need to allow for the collection of energyreleased by the capsule. In one embodiment, the exit apertures may beplaced within one of a small number of cones, called the IFE cones. Asdiscussed below, precise configuration of the beamlets may be set by therequirements for adequate or optimal release of energy from the targetafter irradiation by the beamlets.

In one target concept, known as indirect drive, a spherical target iscontained in a cylinder of radius about 2 mm and height 4 mm, with twosmall holes in its flat sides approximately 1 mm in diameter. The laserlight enters the holes and impinges on the inside surface of thecylinder, causing the release of X-rays which cause the target toimplode and the nuclear fuel contained within the target to releasenuclear energy. For indirect drive, approximately half of the beamletsmay be placed so that their direction of propagation lies in a cone sothat they enter one of the two holes in the cylinder with very littlelight hitting the outside surface of the cylinder. The remainder of thebeamlets may lie in an almost identical pattern in a second cone suchthat they enter the other hole in the cylinder. The two cones may havealmost the same cone angle, their apical points coincident, and theiraxes coincident and oppositely directed. This is known as the two-sidedindirect drive configuration. The two IFE cones typically have ahalf-cone angle of less than 20 degrees so that the total solid angle ofall the beamlet apertures taken together is significantly less than 0.5steradians.

In another target concept, known as polar direct drive, a spherical ornearly spherical target is again contained in a similar cylinder, andthe beamlets are arranged so that they enter the holes, but in thiscase, they impinge directly on the target containing the nuclear fuel.The beamlets are again laid out in two cones so that so that they areevenly divided between two cones, but the cone angles are typicallyconsiderably smaller to ensure that every beamlet impinges directly onthe target. The cone angle for polar direct drive is typically less than10 degrees.

In yet another target concept, known as direct drive, a spherical ornearly spherical target is directly illuminated by the beamlets, and thebeamlets are arranged around the target in a spherical or equivalentlyspherical configuration in which the number of beamlets per steradian ofsolid angle is approximately the same for any group beamlets. Theconfiguration for direct drive may also group the beamlets into severalcones, such that all the cones have the same apex, but their axes aredistributed approximately spherically around the target. A furtherpossible variation on the illumination geometry places the beamlets in asmall number of cones, all with (almost exactly) coincident apices, butwhose axes are not distributed spherically around the target. In thiscase, the sum total of all the pulses delivered by the beamlets causesthe target to execute a controlled approximately spherical implosion,despite the nonspherical arrangement of beamlets around the target.

It is clear that many configurations of beamlets around the target arepossible, and various embodiments may include all possible arrangementsof beamlets.

In order to achieve the efficient release of nuclear energy from thetarget, the sum total of all the pulses typically must meet certainstrict requirements. These originate in the need to accelerate thecompression of the target in an optimal manner, the need to have a smallvolume of fuel at high temperature and pressure at the stagnation point,the need to prevent the growth of deleterious instabilities in thecapsule during compression, and the need to avoid laser plasmainstabilities. These requirements are known as temporal pulse shaping,spatial profile shaping, beam smoothness to avoid the Rayleigh-Taylorand other instabilities, and suppression of laser-plasma instabilities(LPI). Reasonably precise control of the features of the laser lightfrom the beamlets is required.

Typically, previous and proposed approaches to designing driver lasersfor ICF have used a moderate number of beams, such as 192 (NIF) [Haynam2007] or 240 (LMJ) [Bettinger 1999], each one producing a very similarpulse. Each beamlet is tailored precisely to meet the targetrequirements. Given the challenges and constraints of laser technology,compromises must typically be made. In various embodiments, each beamletis independently specified. This results in extreme flexibility inadjusting the features of the total laser drive on the target. Invarious embodiments, it is possible to have simultaneous properties ofthe total drive at the target which are either impossible or verydifficult to deliver from a single large aperture. In the preferredembodiments, this flexibility is exploited to facilitate the bestperformance of any previous or proposed driver laser.

The embodiments described herein take advantage of the extremeflexibility of the embodiments in regard to the beamlets' individualpulse shape, wavelength, bandwidth, temporal sequencing, and focusing.

In the first preferred embodiment, there are 66000 beamlets, dividedinto two cones of 33000 beamlets. The beamlets are configured fortwo-sided illumination using cone angles of 24 degrees. The beamletapertures are packed as close together as practical within the two IFEcones, and respecting the minimum area of each aperture arising fromoptical damage prevention. Each beamlet produces a light pulse that ismonochromatic and spatially uniform or supergaussian at its exitaperture. While not required in general, in this embodiment the temporalpulse shape of the beamlets pulses are the same and timed to arrive atthe target essentially simultaneously. The wavelengths of the individualbeamlets are distributed randomly over a range of 1.5% of their averagewavelength, which is approximately 300-400 nm, or 15 THz. In thisembodiment, the δI/I of the laser intensity at the target drops rapidlyas τ/t, where the time constant τ is about 35 fs, and asymptotically thesmoothness is zero. The total bandwidth of the laser light is about 15THz. This distribution of wavelengths is achieved by selecting suitablelaser gain media for each beamlet individually. Suitable laser gainmedia are those which collectively support 15 THz of bandwidth for theensemble of beamlets that comprise the embodiment. The laser gain mediumfor each beamlet is selected to give the desired parasitic-freeperformance at its assigned wavelength. Ideally, the distribution offrequencies among the beamlets is close to uniform so that there are nosignificant gaps in the total spectrum delivered by the embodiment. Inthis embodiment, the wide range of frequencies is provided by aselection of diode-pumped Nd:Glass laser gain media. Flashlamp anddiode-laser-pumped Nd:Glass gain media are well known in the art of ICFand generally have significant gain bandwidths. The total bandwidth of1.5% can be covered with just a few glass media, typically phosphate,silicate and aluminate glasses, which together operate over a totalwavelength range extending from about 1070 nm down to about 1030 nm.Glass laser media spanning the desired frequency range have beendeveloped recently at Schott, North America under a private contract andare expected to become available in time for use in a future embodimentof the laser system. There is a small loss in efficiency for thosebeamlets not operating at the frequency of peak gain for its gainmedium. However, a beamlet may be operated at a frequency where its gainis half its peak gain without an unacceptable loss in efficiency. Thebeamlets are frequency-converted to the UV for optimal target coupling.

Other choices for laser gain media are possible. There are two types ofceramic laser gain media: (a) transparent crystalline ceramics formed bydense, doped microcrystals, and (b) a transparent glass ceramiccomprising a glass matrix with dispersed microcrystals doped withtransition metals. Such media have been demonstrated at small scale withattractive properties for this application and there is no knownimpediment to making them at the required scale of several centimeters.Another potential gain medium is Nd:SiO₂, Neodymium-doped fused silica.This material has a high damage threshold, a bandwidth in excess of 2%,and a reasonable gain cross-section, so that the material can operatewith adequate efficiency at pulse lengths of several nanoseconds, and atthe same time support a total bandwidth of 15 THz.

Diode-laser-pumped Nd:Crystal laser media are also well-known in the artbut typically have narrower gain bandwidth than Nd:Glass laser media sothat a greater number of different media are required to cover thedesired wavelength range. Achieving a uniform spectrum using crystallinegain media will require many different gain media, possibly based onexisting gain media with additional dopants to shift the peak gainwavelength. The small aperture of each beamlet may enable a larger rangeof laser/host materials to be exploited in our approach. Manufacturingdifficulties associated with large apertures (D>10 cm) are well known inthe ICF community, and methods for developing manufacturing processesare also well-known.

The extraction efficiency of a laser amplifier, and the relationshipbetween the input and output pulse shapes, vary with the opticalfrequency of the laser pulses. Far from the peak of the gain profile,the amplifier transfers energy to the laser pulses inefficientlycompared to the efficiency for operating at that peak. In general,Frantz-Nodvik theory indicates that the efficiency is acceptable foroptical frequencies within the FWHM of the gain profile. In embodimentsof the multiple aperture approach, we envision groups of beamlets eachcontaining the same laser gain medium, but operating at differentoptical frequencies. For example if there 50,000 beamlets and just 10gain media, then there may be 10 groups of approximately 5000 beamletswith the same gain medium. Within any one group, each beamlet mayoperate at a different optical frequency, with the efficiency and pulseshape distortion appropriate to its individual frequency.

There are several approaches to managing the variations in efficiencyand pulse shape distortion among the beamlets. In the preferredapproach, each amplifier in a group is pumped to the same energydensity, and the input pulse energies and pulse shapes are also thesame. Then, the output pulse energy and output pulse shape varythroughout the group. Because there are very many beamlets in the group(˜5000), the variation in the output of each beamlet averages out at thetarget. The gains and pulse shapes are adjusted so that the total driveat the target meets the requirement. Other approaches are also possible,for example, where the pumping and input pulses are individuallyadjusted among the beamlets to provide a more uniform distribution ofpulse formats at the target. There is clearly great flexibility inconfiguring each amplifier to accommodate an optical frequency off thepeak of the gain curve. Accordingly, all such variations and approachesare deemed within the scope of the embodiments.

In an embodiment, each beamlet has a MOPA configuration wherein itslaser oscillator is tuned to provide its assigned infrared wavelength.The output of the infrared laser is frequency-converted to theultraviolet using standard nonlinear-optical techniques such asfrequency tripling in KD*P or frequency doubling in YCOB. The conversionefficiency can be high because the bandwidth of each beamlet is narrowand its beam quality is high in view of the need to focus on a smalltarget several meters away. The laser gain medium and the oscillatorgain medium are matched for each beamlet so that the wall-plugefficiency of each beamlet is not compromised by having a peak gainwavelength somewhat different from the operating wavelength. Even thougheach beamlet has an individual wavelength different from all the others,beamlets with wavelengths close together can use the same gain medium,depending on the gain bandwidth of the available media.

In the embodiment, the pulse at the exit aperture of a beamlet at itsexit aperture is relatively flat temporally, supergaussian spatially,and narrowband. The frequency-conversion efficiency for such pulses isabout 90% to the visible and 80% to the ultraviolet. This is aconsiderable improvement over present and proposed approaches where eachbeam delivers the same shaped pulse, and where the frequency conversionefficiency is around 55%. Thus in various embodiments, the infraredenergy of all the beamlets before frequency conversion is about 70% ofthe infrared energy needed in present and proposed lasers for ICF andIFE.

Because the driver laser will deliver several pulses per second, thelaser amplifiers should be designed to efficiently remove the heatgenerated by the laser amplification process. Crystal lasers typicallygenerate significantly less heat than glass lasers, require lesspowerful pump sources, and are more efficient at transporting heat outof the amplifier. Thus, they can handle the heat more gracefully thanglass lasers. Ceramic glass lasers also have attractive and adequatethermal properties. In practice, there is a trade-off between the numberof gain media and the repetition rate of the driver laser. Othercombinations of gain media are also possible in various embodiments.Glass, ceramic and crystal laser media can be used in differentbeamlets, and this may prove to be an attractive cost-effective optionfor some or all of the beamlets. The small aperture is more readilyamenable to efficient cooling for operation at 5-15 Hz repetition rate.

In another embodiment, the beamlet wavelengths are not distributedrandomly but carefully selected and implemented. This configurationimproves the smoothing rate for long wavelength spatial modes of theintensity near the target. This embodiment exhibits extreme flexibilityof the laser system.

In general, the beamlets approach the target from different directions;two beamlets separated in beam direction by an angle θ have an opticalinterference pattern with period Λ=λ/sin θ. The rate at which thesespatial periods are smoothed depends on the beat frequency between thetwo beamlets. Thus, by carefully selecting the distribution of thefrequencies among the beamlets, the rate of smoothing of the spatialperiods in the total laser drive at the target can be controlled. Thiscontrol does not exist in conventional approaches such as NIF-stylelasers. Long spatial periods are associated with beams that are close inbeam direction, and shorter periods with beamlets that are more distantfrom each other. A distribution of frequencies in which beams withnearly equal frequencies are generally widely separated in beamdirection may smooth the longest spatial frequencies, or lowest L-modesof the total drive, the fastest. If the distribution has beamlets withsimilar frequencies close in beam direction, then the lowest L-modes maynot smooth rapidly. Thus, it is possible to control the most persistentL-modes in the total drive by carefully adjusting the distribution offrequencies among the beamlets. The correlation between frequency andbeam direction is a new feature the embodiment that does not exist inconventional approaches such as NIF-style lasers.

One way to view the correlation between frequency and beam direction isto describe it as a tiling of the K-space of the local field of thedrive. Each beamlet contributes one “tile” to the K-space of the localfield. Each Fourier component I(κ,t) of the intensity at the target is aconvolution of the field with itself. It therefore oscillates at thebeat frequencies of the pairs of beams that contribute to that Fouriercomponent. Different distributions of frequencies among the beamletslead to different frequencies in I(κ,t). Hydrodynamic and laser plasmainstabilities are strongly dependent on the frequency content of I(κ,t),and therefore can be controlled by the distribution of frequencies amongthe beams. This is a fundamental mechanism by which embodiments have theflexibility to control the deleterious instabilities in the ICF target.

Another fundamental mechanism by which smoothing and LPI may becontrolled is simply through the statistical averaging of I(κ,t) over alarge number of beamlets. On any given shot, the absolute phase betweenthe beamlets varies randomly because the absolute phase derives fromquantum noise in the oscillators that provide the seed pulses for thebeamlets. There will be shot-to-shot variations in the laser drive asthe beamlet phases change from shot to shot. With a large number ofbeamlets, both the average value of <I(κ,t)> and its variation from shotto shot are reduced, typically as 1/√N. Shot-to-shot variations inconventional ICF systems use seed pulses that are derived from a singleoscillator and the role of the quantum phase of that oscillator isbenign. Rather, shot-to-shot variation in conventional systems derivesfrom random fluctuations in the control parameters of the laser. With afew hundred beams, the statistical averaging is only a factor of 20 orso, but with several thousand beams, as in the embodiments describedherein, the statistical averaging can be as much as a factor of hundred.This ameliorates the tolerances on the control parameters of thebeamlets, with cost savings and improved stability of the laser drivefrom shot to shot.

In the embodiment, the seeding of hydrodynamic instabilities and thegrowth of laser plasma instabilities are controlled by the smoothingrate and the total bandwidth of the ensemble of beamlets. Both of theseinstabilities depend on the spatial frequencies in the electric field ofthe laser drive, and on their frequency content. Some spatialfrequencies are more problematic than others. For example, thehydrodynamic instabilities are most virulent at spatial scales of 10-30microns, which corresponds to the interference between beamletsseparated by about 10-30 milliradians (roughly 1 degree). By arrangingfor beamlets separated by this angle to have significantly differentfrequencies, the most harmful hydrodynamic spatial frequencies in thedrive can be reduced.

For illustrative purposes, a distribution of the frequencies among thebeamlets that reduces the low L-mode terms in the laser drive is givenin FIG. 15, which illustrates a spatial spectrum of a group of beamlets,according to an embodiment. Consider a hexagonal arrangement of beamletexit apertures. Each beamlet has a spatial spectrum of size kD/f, wherek=2π/λ, D is the beamlet aperture, and f is the focal length of thelenses focusing the beamlet on the target.

The spatial frequency spectrum of the beamlets at the target isrepresented in FIG. 14. FIG. 15 illustrates an exemplary pattern ofspatial frequency spectrum of four frequencies. FIG. 16 illustrates anexemplary pattern of spatial frequency spectrum of nine frequencies.FIG. 17 illustrates an exemplary pattern of spatial frequency spectrumof sixteen frequencies. The “tiling” of the K-space, one tile perbeamlet, is evident. Each circle is an image of a beamlet in K-space.The intensity spectrum is the convolution of this pattern with itself,in a manner well-known to practitioners of the art of ICF. With justfour frequencies arranged in the pattern as shown in FIG. 15, no imageis next to an image of the same frequency.

The separation between any image and the nearest image at the samefrequency is one image diameter. With nine frequencies arranged in thepattern shown in FIG. 16, the separation is two image diameters.

Therefore, it is quite possible to arrange for all the cross-terms inthe convolution to average to zero, for κ up to about twice the size inκ-space of an individual beam image. The convolution representing thespatial spectrum of the intensity has reduced low L-modes.

The pattern of all the 1's or 2's etc. among the beams determines thestrongest spatial modes in the intensity. A distribution which controlsthe spatial frequency represented by all the 1's or 2's etc. can easilybe constructed by using sixteen frequencies instead of nine, asillustrated in FIG. 17.

Clearly there is a vast number of combinations and arrangements of thefrequencies among the beams. The arrangement given here illustrates theprinciples by which the distributions most attractive for beam smoothingcan be discovered. The number of frequencies and their distributionamong the beamlets may depend on the exact values of the angularseparation of the beamlets and the most harmful spatial scales Λ₀. Ingeneral, the minimum number of frequencies required to create thedesired distribution in the illustration is approximately (Λ₀/λ)².However, the asymptotic smoothness is essentially the inverse of thesquare root of the number of different frequencies. Therefore, it may beadvantageous to have at least 10,000 frequencies to meet the asymptoticsmoothness requirement of ICF. If each beamlet is single frequency, asin this embodiment, this would be a primary reason for a large number ofbeamlets. If beamlets are chosen to have several frequencies, the numberof beamlets can be reduced accordingly, with attendant consequences forthe spatial spectrum of the intensity at the target.

Other methods of determining and evaluating useful distributions of thefrequency among the beams, and the implementation of this method will bewell understood by practitioners of the art of ICF, and do not requirefurther elaboration here. All distributions, whether obtained by thismethod or any other, are deemed within the scope of the embodiments.

Moreover, the laser plasma instability known as stimulated Brillouinscattering (SBS) is most harmful for back-scattered light and depends onthe persistence of a density grating in the plasma. SBS is relativelyinsensitive to the spatial frequencies in the laser drive, but it isvery sensitive to the total bandwidth. Bandwidths as small as 1 THz canbe effective in controlling backward SBS. Embodiments are capable ofbandwidth of 10-50 THz subject to the availability of suitable lasergain media, and so offer the possibility of eliminating SBS in thetarget through the total bandwidth. Similarly, stimulated Ramanscattering (SRS) is most harmful for side-scattered and back scatteredlight, and depends on the persistence of a grating in the electrondensity of the plasma. It is sensitive to bandwidth just as SBS is, butits suppression requires bandwidths well in excess of 5 THz. Embodimentsalso offer the possibility of controlling SRS through total bandwidth.

The 2ω_(pe) instability occurs at exactly quarter-critical electrondensity, which occurs in a small region of the plasma, which depends onthe laser frequency. If the laser has bandwidth, the location where theinstability occurs is spread out, and the growth rate of the instabilityis reduced. Embodiments have the capability to achieve bandwidths wellin excess of any other ICF driver laser, and are capable of suppressingthis instability through the total bandwidth. Other harmful effects inthe plasma such as hot electron production can also be controlled bybandwidth through essentially the same mechanism of spreading out theregion of the plasma where these effects occur.

A full description of these instabilities and the effect of bandwidthand the smoothing of the spatial frequency spectrum on them is omittedbecause the many theoretical models and experimental tests of theseinstabilities are well-known to practitioners of ICF and they will beable to calculate the benefits the embodiments in regard to the highsmoothing rates, low asymptotic smoothness, and LPI suppression.

The benefits of the embodiments and their flexibility have beendescribed here in general terms readily appreciated and evaluated bypractitioners of the art of ICF. In the embodiments, total bandwidthcontrols the rate of smoothing and suppresses deleterious plasmaeffects, and the distribution of frequencies among the beams controlsthe smoothing rate and the asymptotic smoothness. The description givenhere, while somewhat foreshortened, and the methods of discoveringuseful distributions will nonetheless be adequate to guide practitionersof the art of ICF in the beneficial use of the invention as defined bythe following claims. [See for example, Lehmberg 2000].

The use of very many beamlets provides cost and operations advantages.The beamlets may share power supplies, control systems, and diagnosticequipment which reduces the mass of the support systems required tooperate the driver laser system and therefore reduces the cost andoperational complexity. The beamlets may be physically grouped whereeach group can be independently changed out for maintenance,replacement, and refurbishment of the laser hardware. Target irradiationoperations may not require that the IFE cone be fully filled. Removing asmall number of beamlets from an IFE cone can be compensatedstraightforwardly by adjusting the power of the remaining beamlets. Thebeamlets can therefore be arranged in groups so that turning off anygroup can be compensated by adjusting the output of the remainingbeamlets in the IFE cone. This enables preserving driver laserperformance during maintenance operations. With very many beamlets, theequipment overhead required to maintain continuous target irradiationsoperations during maintenance operations is small, thus reducing thecost of the driver laser system. The maintenance groups can consist ofas few as one beamlet, or a small number of beamlets.

In another embodiment, the beamlets are time-sequenced so that only aportion of the beamlets pulses irradiate the target at any one time. Thefirst few nanoseconds of the target drive are provided by about 5,000beamlets, each producing the same temporal pulse shape of about threenanoseconds in pulse length, the same wavelength, and minimal(individual) bandwidth. This group of lasers may operate in theinfrared, with no frequency conversion to the ultraviolet, and has a(group) bandwidth of about 1%. The bandwidth suppresses the imprintingof spatial nonuniformity in the target. Laser-plasma instabilities arenot present at this time in the target response due to the size of theplasma and the low intensity on target, so no means of suppressing themis required. During the next approximately twenty nanoseconds, about35,000 beamlets deliver pulses to the target. Each beamlet's pulse istemporally shaped so that the total laser power at the target changessteadily over time, generally increasing to provide the desiredimplosion time history for the target capsule. The wavelengths of thebeamlets are initially in the infrared, but as the implosion processproceeds, the pulses are delivered from frequency-converted beamlets sothat the wavelength of the laser light at the target moves from theinfrared to the ultraviolet. Some beamlets are converted to the secondharmonic in the visible; others are converted to the third harmonic inthe ultraviolet. The group bandwidth of the beamlets operating in theinfrared remains at about 1%. The group bandwidth of the beamletsoperating in the visible may be smaller but the group bandwidth of theultraviolet beamlets is about 1.5%. The beamlets operating later in thetarget implosion are focused a little behind the beamlets operatingearlier in the target implosion process (i.e., they have longer focallength), and their spot size is smaller. This compensates for any changein size of the target during the implosion process (so called beamzooming). Finally, the remaining 26,000 beamlets deliver their pulses tothe target. Each beamlet in an IFE cone delivers a flat temporal pulseshape of about 3-5 ns in pulse length, is focused to a common point, andis frequency-converted to the ultraviolet. The total (group) bandwidthis about 1.5%. Thus the color, bandwidth, focal length, and focal spotsize of the complete laser drive at the target are finely adjustedduring the target implosion process to optimize the laser drive pulseshape at the target, while avoiding hydrodynamic and laser-plasmainstabilities.

Clearly, any property of a laser beam can be distributed among thebeamlets. Many combinations are possible, with different propertiesdistributed differently. Beamlets close in frequency may have a range offocal lengths, polarizations, and pointing, so that these properties ofthe total drive are not correlated to frequency. Or beamlets close infrequency may be assigned the same focal length, and/or polarization,providing a high degree of correlation with frequency. The distributionof beam properties among the beamlets and the distribution of frequencyamong the beamlets may control the property's temporal and spatialcharacteristics. The number of permutations of the beamlet parameters isvirtually unlimited.

In this way, the embodiments maximize the release of nuclear energy,minimize the size of the driver laser, and optimize the ratio of nuclearenergy released to the total laser energy delivered to the target.

These two embodiments represent the simplest implementations of variousembodiments, and a more complex implementation of various embodiments.The benefits described are enabled by the use of many small beamlets togive flexibility, cost effectiveness and operational efficiency to anICF driver laser or an IFE driver laser. Clearly there are manyembodiments that take advantage of having very many beamlets and utilizeone or more of the methods described herein. All of these embodimentsare deemed to lie within the scope of the invention as defined by thefollowing claims. However, the embodiments should not be construed aslimiting, because the disclosed embodiments are only exemplary andprovided to illustrate the principles of the invention to theunderstanding of those of ordinary skill in the art.

FIG. 2 illustrates elements of a driver laser and reactor for an IFEpower plant and the general path followed by a pulsed laser beam in theIFE power plant. The laser pulse is generated in an oscillator 200,amplified in a series of power amplifiers and transport optics 201,passes through a beam focusing and conditioning optic 203, and entersthe IFE reactor target chamber 204. The laser pulse may be frequencyconverted in a frequency convertor 202. The result of the laser pulseimpinging on a target is target capsule implosion and release of energyby the target, 205. The oscillator 200 creates a small energy laserpulse, typically in the range of nanojoules, with the desired temporaland spatial features. The power amplifiers and transport optics 201increase the energy of the pulse and maintain adequate beam quality sothat the beam focuses tightly downstream on the target. The frequencyconvertor 202, if present, converts the wavelength of the laser pulse asfar to the ultraviolet as is required by the target, typically 250-550nm. The beam focusing and conditioning optics 203 focuses the beam ontothe target and also adds carefully designed spatial structure so thatthe spot size at the target matches the target size and also provides asmoothly tailored spatial profile to control the low L-mode terms in theimplosion pressure on the target. The laser may be configured to deliverpulses to the target at a rate up to about 20 Hz.

FIG. 3 illustrates a spherical arrangement of beamlets 410 around atarget chamber, whose wall is denoted 430. An interior region of thetarget chamber 420 may be at a high vacuum, typically 0.01 torr, andcontain the target 440 nominally at its center. As FIG. 3 illustrates,some portions of the laser subsystems comprising the beamlets 410 resideon an interior side of the target chamber wall 430 in the interiorregion of the target chamber 420, but other portions of the lasersubsystems comprising the beamlets 410 remain outside the target chamberwall 430. FIG. 3 shows the beamlets 410 arranged in a symmetricalarrangement so that the beamlets 410 occupy positions spanning the full4π solid angle available at the target 440. Any portion of solid anglenot occupied by any beamlet 410 may be used to collect the high energyparticles released by the target implosion. The spherical arrangement isa preferred arrangement for direct drive targets.

FIG. 4 illustrates a conical arrangement of beamlets 410 around a targetchamber 430, where the beamlets 410 are grouped into two clusters 450and 460. The two clusters 450 and 460 may have a conical shape. Theclusters 450 and 460 may each fill a circle as viewed from the target440, or the clusters 450 and 460 may each fill a square arrangement. Thepreferred arrangement is circular so that each of elements 450 and 460represents a mathematical cone which contains all the beamlets 410 inthe respective cluster 450 and 460. The main advantage of the conicalarrangement is the increased efficiency of collection of the energeticparticles released by the target in implosion.

These arrangements of beamlets 410 around a target chamber 430 shouldnot be construed as limiting. In various embodiments, other arrangementsof the beamlets 410 are also possible.

FIG. 5 illustrates details of the relationship between a beamlet 410 andthe target chamber wall 430, according to an embodiment. Ports 405 arepositioned in the target chamber wall 430 through which the beamlets 410pass. The ports 405 may be windows in the target chamber wall 430 thatare optically transparent, but strong enough to contain high vacuum. Thewindows may be protected from the energetic particles released by thetarget by positioning a shield in front of the windows. The beamlets 410enter the target chamber interior 420 at an angle to the chamber wall430 and are directed to the target 440 by a grazing incidence mirror ina manner well-known to practitioners of this art. While the shields andmirrors are not explicitly shown in FIG. 5, the ports 405 are consideredto represent them along with the windows.

FIG. 6 illustrates elements of a driver laser for inertial fusionenergy, according to an embodiment. The driver laser may include aplurality of beamlets 600. The beamlet 600 may be an embodiment of thebeamlet 410. The main subsystems of the beamlet 600 include a lasersystem 610 which outputs a rectangular beam 615, a beam reshaper 620that includes bi-cylindrical lenses and outputs a square beam 625, abeam rotator 630 including two out-of-plane mirrors that outputs arotated square beam 635, beam positioning and pointing optics 640 thatpoint the rotated square beam 635 toward a final focusing lens 650 thatin turn outputs a focused rotated square beam 655 to a phase plate 660.The phase plate 660 includes square segments and is rotated to align itssquare segments with the rotated square beam profile to output aphase-adjusted beam 665 through a debris shield 670 toward the target440. The port 405 shown in FIG. 5 divides the subsystems of the beamlet410 into those at high vacuum inside the target chamber wall 430 andthose outside the target chamber wall 430. The target chamber wall 430may be after the beam positioning and pointing optics 640, or the targetchamber wall 430 may be further downstream. The elements of the beamlet410 should be placed in relation to the target chamber wall 430 suchthat the laser system 610 and any sensitive optics of the beamlet 410are protected from the high temperature of the target chamber 430 andthe neutron flux and debris released by the target 440 afterirradiation.

FIG. 7 illustrates a schematic diagram of a beamlet 410 for a driverlaser for inertial fusion energy, according to an embodiment. Anoscillator 710, corresponding to the oscillator 200 of FIG. 3, outputs alaser pulse to power amplifiers 711 and transport optics 712. Afrequency convertor 713 receives the amplified laser pulse from thetransport optics 712, converts the frequency of the laser pulse, andoutputs the frequency-converted laser pulse to a beam reshaper 714. Thebeam reshaper 714 reshapes the beam shape of the laser pulse from arectangular to a square aperture and outputs the reshaped beam to targetchamber transport optics 715 to send the reshaped beam through thetarget chamber wall 430 into the target chamber interior 420. Inside thetarget chamber 420, the reshaped beam is turned by a final turningmirror 716 to go through a phase plate 717. The phase plate 717 isconfigured to control the properties of the focal spot of the beamlet410. After the laser pulse passes through the phase plate 717, a finalfocusing lens 718 focuses the laser pulse onto the target 440. It ishighly desirable that the only optical element that is exposed to theproducts of the capsule implosion of the target 440 is the final turningmirror 716. In the preferred embodiments described herein there is onlyone each of the power amplifiers 711 and transport optics 712.Protection of the optics 716 and 717 is accomplished with a solid shieldbetween the optics and the center of the target chamber.

FIG. 8 illustrates a total power of all beamlets 410 in a driver laseras a function of time, according to an embodiment. In the variousembodiments, the driver laser includes very many beamlets 410, each ofwhich is a simple laser system. FIG. 8 illustrates an exemplary targetirradiation pulse 802 and an exemplary history of the target shellradius vs. time 801 to show the advantages of having very many beamlets410 in the driver laser. The target irradiation pulse 802 generallyincreases in power up until about a time at which the target shellradius stagnates during a typical target implosion. The curve of thetarget shell radius vs. time 801 shows the target shell radiusaccelerating inward and stagnating at about 1/40 of its initial radius.At the point of stagnation, the nuclear payload in the target capsule440 ignites and burns, and the target shell radius rapidly acceleratesoutward as the target shell disassembles.

The target irradiation pulse 802 represents the contributions of all ofthe beamlets 410. The target irradiation pulse 802 increases in powerwith time and shuts off at or a little before the time when the targetradius stagnates. The target irradiation pulse 802 is divided into threesegments: an initial segment 803, a growth segment 804, and a high powersegment 805. The drive in each of these segments is the sum of thedrives from all of the beamlets 410. It will be understood by thepractitioners of this art that the boundaries between these threesegments 803,804, and 805 are not rigidly fixed, but represent thegeneral boundaries where the influences of different aspects of thetarget irradiation pulse 802 have the most significance.

In pulse segment 803, the laser beam impinges directly on the targetcapsule 440 initially, so any spatial nonuniformities in the laser beamare transferred directly to the shell of the target capsule 440. Thesenonuniformities can be expected to develop and grow as the shell isaccelerated inward. Minimizing the imprint of the laser beam on thetarget capsule 440 is a high priority during segment 803. The uniformityof the laser beam can be optimized by having approximately 5,000separate beamlets 410, each with a different frequency spanning abandwidth of about 1.5% of the mean laser frequency, or about 15 THz infrequency. The smoothness of the laser drive (i.e., summation of allindividual laser beams from the individual beamlets 410) on the targetcapsule 440 drops very rapidly, and asymptotes to a very small number,for example, much less than 1%.

FIG. 9 illustrates smoothness of the laser drive at the target as afunction of time, according to an embodiment. FIG. 9 also shows how fastthe nonuniformity of the laser drive decreases in time in general foreach of the compared types of lasers. Here (δI/I) is the nonuniformity,t is time in picoseconds, and Δω_(rms) is the total bandwidth spanned bythe 5,000 beamlets. FIG. 9 compares the smoothing of an embodiment asdescribed herein with the performance of the other smoothing schemes SSDand ISI. Note that FIG. 9 compares schemes using laser systems of thesame total bandwidth, and not for actual laser systems that utilizethese smoothing schemes, because different laser systems have differenttotal bandwidth.

FIG. 10 illustrates the smoothness of the laser drive at the target as afunction of time, comparing prior driver lasers with an embodiment asdescribed herein. FIG. 10 compares the smoothing of an embodiment of a1.5% bandwidth multiple aperture laser with the performance of Omega[LLE201] and Nike[Deniz 1998]. The smoothing performance of NIF[Haynam 2007]is significantly worse than Omega. The clear advantage of the multipleaperture system is evident in FIG. 10 due to the better smoothnessachieved. Moreover, the achievable bandwidth for multiple aperturelasers is limited only by the availability of lasers with a suitablewavelength, and is not intrinsically limited by the smoothing technique.

The spectrum of the intensity ripples of the laser drive at the target440 can be controlled by selecting the mean wavelength of the beamlets410 and by controlling the distribution of wavelengths among thebeamlets 410. There is some advantage to having the wavelength of theintensity ripples be shorter rather than longer. This can be achieved byhaving a mean laser wavelength in the ultraviolet and by arranging forthe distribution of frequencies among the beamlets 410 so that beamlets410 with frequencies that are close to each other have apertures thatare far apart from each other. This technique is called color separationand is illustrated in FIGS. 11, 12, and 13, as described below.

FIG. 11 illustrates an optimal distribution of four frequencies amongthe beamlets 410, according to an embodiment. FIG. 11 shows a section ofthe of the target chamber wall 430 as seen by the target 440. FIG. 11also illustrates a plurality of beamlet apertures 120 represented asrectangular in shape, but this restriction is not necessary and shouldnot be construed as limiting, as the beam apertures 120 may take theform of other shapes. Some of the apertures 120 in are labeled by anumber designating the wavelength of the aperture, for example,wavelength 1, wavelength 2, wavelength 3, and wavelength 4. Thedistribution shown involves only four different wavelengths, but clearlythe separation between apertures of the same wavelength is at least twoapertures. If the apertures were square or circular the wavelengthseparation would be even better.

FIG. 12 illustrates an optimal distribution of five frequencies amongthe beamlets, according to an embodiment. FIG. 12 shows a similararrangement as shown in FIG. 12, but using five wavelengths instead.

FIG. 13 illustrates an optimal distribution of nine frequencies amongthe beamlets, according to an embodiment. FIG. 13 shows a similararrangement as shown in FIGS. 11 and 12, but using nine wavelengthsinstead. Clearly by using very many different wavelengths, it ispossible to arrange for the wavelength separation between similarwavelengths to approach one half the total spread of the apertures.Although we have described herein an approach to maximizing wavelengthseparation, this should not be construed as limiting, because thetechniques described herein will be recognized by one of ordinary skillin the art as allowing great flexibility in controlling the spatialripples in the intensity pattern at the target 440.

For the high power segment of the laser pulse 802 in FIG. 8, the plasmasurrounding the capsule is well-developed and LPI are significant atquarter-critical and lower density in the plasma. LPI can be completelysuppressed by configuring the driver laser as approximately 26,000beamlets 410 containing a few widely separated frequencies. Theseparation in frequency should be greater than the growth rate of theLPI. The number of different frequencies should be greater than thefactor by which the total laser intensity from all beamlets 410 exceedsthe LPI threshold intensity. It is expected that four or five differentfrequencies will be adequate to completely suppress LPI. The meanwavelength of the beamlets 410 should be in the ultraviolet to optimizethe coupling of the laser pulses to the target capsule 440. If a longermean wavelength is used, the number of different frequencies can beincreased to compensate for the associated decrease in LPI threshold,and the increased LPI gain. Alternatively, LPI can be eliminated byhaving a distribution of frequencies among the 26,000 beams whose totalbandwidth is several times the growth rate of the LPI, about 50 THz.

For the growth region of the laser pulse 804, the requirements onsmoothing and LPI are both less stringent than in the regions 803 and805. Therefore, the target requirements can be met in the region 804 byhaving a distribution of frequencies that meets the requirements ofregions 803 and 805 simultaneously. One solution is to have severalgroups of beamlets 410, where the bandwidth of each group is of order 15THz, but the total bandwidth of all the groups together exceeds about 50THz.

The pulse length and pulse shape of each beamlet 410 may be chosenindividually to optimize the performance of the beamlet 410. Forexample, the pulse shape may be flat in time to optimize the wall-plugefficiency of the laser and the efficiency of frequency conversion. Thebeamlet fluence is related to the beamlet aperture through the damagefluence, and the optimum pulse length may be determined by the desiredintensity. The number of beamlets 410 delivering pulses at any time inthe laser drive pulse is given approximately by N τ/T, where N is thetotal number of beamlets 410, τ is the beamlet pulse length, and T isthe laser drive pulse length. The smoothing requirement provides a lowerlimit on the number of beamlets 410 operating at any one time to about5,000, so the individual pulse length must be greater than about 1 ns.The temporal overlap between beamlet pulses as one turns off and anotherturns on places an upper limit on the jitter of each beamlet 410 ofabout 250 ps. The focal spot size and focal spot location of eachbeamlet 410 may be individually selected to allow the total laser driveto follow the target shell radius as the target capsule 440 implodes.Beamlets 410 operating earlier in the laser drive pulse will have alarger spot size and a shorter focal length than those operating laterin the laser drive pulse. A practitioner in the art having ordinaryskill will recognize that these temporal requirements and focalbehaviors are well within current technological capability.

Laser materials exist to provide the desired total bandwidth. In thefield of Nd:Glass and Nd:Crystal lasers, there is wide tunability of thelaser wavelength depending on the host material. For example, a suite ofphosphate, silicate and aluminate glass hosts is known to span theregion 1.025-1.075 microns, which would provide a total bandwidth ofabout 5%, which amounts to 50 THz after frequency conversion to theultraviolet range. These materials may be pumped by diode lasers, whichenable wall-plug efficiencies in the range of 5%-25%. The average powerrequirement for IFE can easily be met using these hosts.

FIG. 18 illustrates a controller 1800 of the driver laser, according toan embodiment. Embodiments of the controller 1800 may control theindividual beamlets of the driver laser, the driver laser as a whole, ora combination thereof. For example, one embodiment of the controller1800 may control each of the beamlets, while another embodiment of thecontroller 1800 may control the driver laser as a whole by communicatingwith each of the embodiments of the controllers 1800 that control thebeamlets.

The controller 1800 may receive input commands remotely over a datacommunications network 1870 or input from a user via the control panel1880, such as turning any of the components of the driver laser orbeamlets on or off, selecting an operation mode, setting a desired laserpulse repetition rate, setting a desired laser pulse shape, setting adesired laser pulse power, setting a desired aiming direction and focalpoint of a laser beam, and setting a desired laser pulse timing of onebeamlet in relation to the other beamlets. The controller 1800 may alsoadjust a wavelength of a wavelength tunable laser. The controller 1800may output information to the user regarding an operational status ofthe beamlets or driver laser using a display panel of the control panel1880 or remotely over the data communications network 1870.

The controller 1800 may include a processor 1810 that performscomputations according to program instructions, a memory 1820 thatstores the computing instructions and other data used or generated bythe processor 1810, and a network interface 1840 that includes datacommunications circuitry for interfacing to the data communicationsnetwork 1870. The data communications network 1870 may include anEthernet network, asynchronous transfer mode (ATM) network, WiFinetwork, IEEE-488 interface bus, universal serial bus (USB), RS-232serial interface, or other communication links and networks as known inthe art. In addition, the network interface 1840 may include a networknode of the data communications network 1870 or electronics configuredto implement protocols of the data communications network 1870. Theprocessor 1810 may include a microprocessor, a Field Programmable GateArray, an Application Specific Integrated Circuit, a custom Very LargeScale Integrated circuit chip, or other electronic circuitry thatperforms a control function. The processor 1810 may also include a statemachine. The controller 1800 may also include one or more electroniccircuits and printed circuit boards. The processor 1810, memory 1820,and network interface 1840 may be coupled with one another using one ormore data buses 1860. The controller 1800 may communicate with andcontrol various sensors and actuators 1890 of the driver laser orbeamlets via a control interface 1850.

The controller 1800 may be controlled by or communicate with acentralized computing system, such as one in a control center of acommercial electrical power plant. The controller 1800 may providenetwork monitoring, power control, remote operation, failure monitoring,and data transfer functions. The controller 1800 may provide additionalcommunications using an RS-232 communications interface and/or aninfrared data port, such as communications with a personal computer(PC). Such additional communications may include real-time monitoring ofoperations of the driver laser or beamlets, long-term data retrieval,and control system software upgrades. In addition, the control interface1850 may include a serial peripheral interface (SPI) bus that may beused to communicate between the controller 1800 and motor controllerswithin the driver laser or beamlets.

The controller 1800 may poll the sensors of the sensors and actuators1890 at a minimum rate such that all data required to control theperformance of the driver laser or beamlets may be obtained by thecontroller 1800 in time for real-time operation of the driver laser orbeamlets. The polled values may be reported by the controller 1800 viathe I/O interface 1830 and/or the network interface 1840. The polledvalues may also be used in control algorithms by the controller 1800,and may be stored to long-term memory or a data storage medium for laterretrieval and analysis.

FIG. 19 illustrates a method of driving an inertial confinement fusionreaction for inertial fusion energy generation, according to anembodiment. While the steps of the embodiment are illustrated in asequential order, this should not be construed as limiting, as invarious embodiments, any of the steps may be performed in a differentorder with respect to the other steps.

In a step 1910, a plurality of laser pulses are emitted from a pluralityof pulsed lasers. In various embodiments, there may be at least 512(i.e., 2⁸) pulsed lasers, or at least 2⁹, at least 2¹⁰, at least 2¹¹, atleast 2¹², at least 2¹³, at least 2¹⁴, at least 2¹⁵, at least 2¹⁶-1, orat least any number within a range of the aforementioned minimumnumbers. In various embodiments, there may also be a maximum of 2⁸+1pulsed lasers, a maximum of 2⁹ pulsed lasers, a maximum of 2¹⁰ pulsedlasers, a maximum of 2¹¹, a maximum of 2¹², a maximum of 2¹³, a maximumof 2¹⁴, a maximum of 2¹⁵, a maximum of 2¹⁶, a maximum of 2¹⁷, a maximumof 2¹⁸, a maximum of 2¹⁹, or a maximum of any number within a range ofthe aforementioned maximum numbers.

A central optical wavelength of each pulsed laser may be between about250 nm and 2500 nm, and the root mean square bandwidth of the lasersystem may be greater than about 1 THz. The plurality of pulsed lasersmay be configured to output laser pulses at a plurality of differentfrequencies. Optical frequencies of at least two of the pulsed lasersmay be different from each other by more than about 1 THz. In addition,optical states of polarization of at least two of the pulsed lasers maybe substantially different from one another.

In a step 1920, the plurality of laser pulses are directed toward atarget from different exit apertures along different propagationdirections such that each of the plurality of laser pulses irradiatedifferent portions of the target within a same time window as the othersof the plurality of laser pulses. In other words, such that all of theplurality of laser pulses irradiate different portions of the targetwithin a same or common time window. The time window may be less thanabout 100 ns, or other period of time sufficient to meet the laser driverequirements to cause the target capsule to implode, heat, and initiatea controlled thermonuclear fusion reaction as discussed herein. Thecontrolled thermonuclear fusion reaction may release energy, forexample, greater than about 20 kJ. Each of the plurality of laser pulsesmay be considered to irradiate different portions of the targetsubstantially simultaneously. The pulses may be considered to irradiatethe target substantially simultaneously when the pulses irradiate thetarget within a close enough window of time such that the targetimplodes to create a controlled fusion reaction as described herein,even though the pulses may not irradiate the target literallysimultaneously. For example, the pulses may irradiate the targetsubstantially simultaneously such that the target implodes while alsoirradiating the target at substantially different times such the pulsesare time sequenced from individual pulsed lasers or groups of pulsedlasers so that all of the features of the driver laser may be finelycontrolled at each instant during the entire laser drive pulse at thetarget. Thus, the pulsed lasers that generate the laser pulses may betime-sequenced so that only a portion of the laser pulses irradiate thetarget at any one precise moment in time.

The laser pulses may be directed toward the target in a substantiallyspherical distribution, in groupings of conical distributions, or inother distributions that facilitate target implosion as describedherein. In addition, the laser pulses may be directed toward the targetsuch that the pulses irradiate the target in an overlapping patternwhere each beam associated with each pulse irradiates less than thesurface area of the target visible to the pulsed laser emitting thebeam, and the beams of the different pulses do not fully overlap. Anexample of an overlapping pattern is a hexagonal pattern as illustratedin FIG. 14. In addition, the laser pulses may be directed toward thetarget in a pattern such that an angle between propagation directions ofany two laser pulses from respective pulsed lasers whose centerfrequencies differ by less than about 250 THz is greater than about 0.01radians. A total area of the exit apertures of the plurality of pulsedlasers may occupy a solid angle of less than 0.5 steradians from thetarget's perspective.

In a step 1930, a distribution of the optical frequencies of the pulsedlasers are correlated with a direction of propagation and a focal spotlocation of the respective laser pulses emitted by the pulsed laserstoward the target according to a predetermined prescription. Accordingto the predetermined prescription, a spatial uniformity of intensity ofthe plurality of laser pulses at a surface of the target as computedfrom a ratio of the root mean square variation in the intensity over thesurface of the target to the average value of the intensity over a timeinterval during which the plurality of laser pulses irradiate the targetsurface may be substantially maximized. Also according to thepredetermined prescription, a variation in spatial uniformity ofintensity of the plurality of laser pulses at a surface of the target ascomputed from a ratio of the root mean square variation in the intensityover the surface of the target to the average value of the intensityover a time interval during which the plurality of laser pulsesirradiate the target surface may be less than about 0.25%. Furthermore,according to the predetermined prescription, a smoothing rate of thesummation of the plurality of laser pulses of the laser system at thetarget may be substantially maximized at a spatial scale length ofbetween about 10 and 100 microns. In addition, according to thepredetermined prescription, the plurality of laser pulses that irradiatethe target may be substantially smoothed at a rate faster than about 30THz.

In a step 1940, a distribution of optical pulse shapes of the pulsedlasers may be correlated with a distribution of propagation and a focalspot location of the respective laser pulses emitted by the pulsedlasers toward the target according to a predetermined prescription.According to the prescription, temporal pulse shapes of at least two ofthe pulsed lasers having different optical frequencies may besubstantially different from one another. As such, the pulses may alsoirradiate the target at substantially different times, as the pulseshape of one laser pulse may not have substantial energy at a moment intime at which the shape of another laser pulse does have substantialenergy, and vice versa. In an embodiment, one laser pulse may have atemporal pulse width of less than about 50 ps, while a different laserpulse may have a temporal pulse width of between 1 ns and 100 ns.

In summary, concepts of the embodiments as described herein may be usedby a practitioner of the art of laser science and technology to providea driver for laser-driven ICF which has the capability and theflexibility to meet the target requirements for IFE, as well as the costand efficiency requirements for IFE power plant for commercialelectricity generation. Embodiments include many small beamlets,together with a distribution of frequencies and other laser beamproperties among the beamlets to optimize beam smoothing and LPIsuppression, a pulse shape output from each beamlet which enables finecontrol of the temporal properties of the laser drive at the target.

In contrast to current lasers for ICF drivers, various embodiments asdescribed herein can deliver greater bandwidth, higher efficiency,greater control of the beams at the target, greater control of theinstabilities which degrade target performance, and more reliable,repeatable and controllable laser performance than any previouslyarticulated approach to ICF drivers. In comparison to all the priordriver lasers, embodiments as described herein exceed their performance.In contrast to the prior driver lasers, embodiments as described hereinhave the flexibility to meet all the requirements for ICF in a fullycontrolled and repeatable manner.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

For the purposes of promoting an understanding of the principles of theinvention, reference has been made to the embodiments illustrated in thedrawings, and specific language has been used to describe theseembodiments. However, no limitation of the scope of the invention isintended by this specific language, and the invention should beconstrued to encompass all embodiments that would normally occur to oneof ordinary skill in the art. The terminology used herein is for thepurpose of describing the particular embodiments and is not intended tobe limiting of exemplary embodiments of the invention. In thedescription of the embodiments, certain detailed explanations of relatedart are omitted when it is deemed that they may unnecessarily obscurethe essence of the invention.

The apparatus described herein may comprise a controller including aprocessor, a memory for storing program data to be executed by theprocessor, a permanent storage such as a disk drive, a communicationsport for handling communications with external devices, and userinterface devices, including a display, touch panel, keys, buttons, etc.When software modules are involved, these software modules may be storedas program instructions or computer readable code executable by theprocessor on a non-transitory computer-readable media such as magneticstorage media (e.g., magnetic tapes, hard disks, floppy disks), opticalrecording media (e.g., CD-ROMs, Digital Versatile Discs (DVDs), etc.),and solid state memory (e.g., random-access memory (RAM), read-onlymemory (ROM), static random-access memory (SRAM), electrically erasableprogrammable read-only memory (EEPROM), flash memory, thumb drives,etc.). The computer readable recording media may also be distributedover network coupled computer systems so that the computer readable codeis stored and executed in a distributed fashion. This computer readablerecording media may be read by the computer, stored in the memory, andexecuted by the processor.

Also, using the disclosure herein, programmers of ordinary skill in theart to which the invention pertains may easily implement functionalprograms, codes, and code segments for making and using the invention.

The invention may be described in terms of functional block componentsand various processing steps. Such functional blocks may be realized byany number of hardware and/or software components configured to performthe specified functions. For example, the invention may employ variousintegrated circuit components, e.g., memory elements, processingelements, logic elements, look-up tables, and the like, which may carryout a variety of functions under the control of one or moremicroprocessors or other control devices. Similarly, where the elementsof the invention are implemented using software programming or softwareelements, the invention may be implemented with any programming orscripting language such as C, C++, JAVA®, assembler, or the like, withthe various algorithms being implemented with any combination of datastructures, objects, processes, routines or other programming elements.Functional aspects may be implemented in algorithms that execute on oneor more processors. Furthermore, the invention may employ any number ofconventional techniques for electronics configuration, signal processingand/or control, data processing and the like. Finally, the steps of allmethods described herein may be performed in any suitable order unlessotherwise indicated herein or otherwise clearly contradicted by context.

For the sake of brevity, conventional electronics, control systems,software development and other functional aspects of the systems (andcomponents of the individual operating components of the systems) maynot be described in detail. Furthermore, the connecting lines, orconnectors shown in the various figures presented are intended torepresent exemplary functional relationships and/or physical or logicalcouplings between the various elements. It should be noted that manyalternative or additional functional relationships, physical connectionsor logical connections may be present in a practical device. The words“mechanism”, “element”, “unit”, “structure”, “means”, and “construction”are used broadly and are not limited to mechanical or physicalembodiments, but may include software routines in conjunction withprocessors, etc.

The use of any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. Numerous modifications and adaptations will bereadily apparent to those of ordinary skill in this art withoutdeparting from the spirit and scope of the invention as defined by thefollowing claims. Therefore, the scope of the invention is defined notby the detailed description of the invention but by the followingclaims, and all differences within the scope will be construed as beingincluded in the invention.

No item or component is essential to the practice of the inventionunless the element is specifically described as “essential” or“critical”. It will also be recognized that the terms “comprises,”“comprising,” “includes,” “including,” “has,” and “having,” as usedherein, are specifically intended to be read as open-ended terms of art.The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless the context clearly indicates otherwise. In addition, itshould be understood that although the terms “first,” “second,” etc. maybe used herein to describe various elements, these elements should notbe limited by these terms, which are only used to distinguish oneelement from another. Furthermore, recitation of ranges of values hereinare merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein.

GLOSSARY

USD: US Dollars

MW: Megawatt

GW: Gigawatt

Cms: centimeters

L-mode: a spatial variation in the drive experienced by the target withapproximately 2 L+1 peaks and valleys around a target perimeter.

Hz: Hertz

ICF: Inertial Confinement Fusion

IFE: Inertial Fusion Energy

LPI: Laser plasma instabilities

Hr: Hour

MW-hr: megawatt-hour

nm: nanometer

kJ:—kiloJoule

ns: nanosecond

ps: picosecond

DT reaction: nuclear reaction between deuterium (D) and tritium (T)nuclei

KrF lasers: A gas laser utilizing Krypton Fluoride as the active medium:

mrad: milliradians

Nd: glass laser: a solid state laser utilizing Neodymium ions as theactive ion in a glass host.

Nd:crystal laser:: a solid state laser utilizing Neodymium ions as theactive ion in a crystalline host

FWHM: full width at half maximum

LPI: laser-plasma instabilities

Target: The device at which a laser beam or combination of laser beamsis directed.

Target Region: A small volume of space enclosing said target.

Target Reference Surface: The surface of said target region.

Intensity: The irradiance of light, often expressed in units ofGigaWatts per square centimeter.

Exit Aperture: The last optical element of a laser.

Beamlet: A laser system which is part of a large laser system, whoseexit aperture is not shared by any other beamlet of said laser system.

Pulse Length: The interval in time in which the intensity of a laserbeam is substantially different from zero.

Pulse Shape: A specific variation of the intensity or total power of alaser beam or laser system within the pulse length of said laser beam orsystem.

Wavelength: The optical wavelength of light.

Bandwidth: The spread in wavelength of either a single laser beam orbeamlet, or the spread in wavelength of a laser system comprising morethan one individual laser.

Bandwidth is often expressed as the ratio of the root mean squarevariation in the wavelength within a specified time interval, to theaverage value of the wavelength over said time interval.

Smoothness: The spatial uniformity of the intensity or fluence of alaser beam or combination of laser beams at a target region surface.Smoothness is often expressed as the ratio of the root mean squarevariation in the intensity over the target region, to the average valueof the intensity over said time interval and at different spatialfrequencies

Substantially the same frequency: Beams of substantially the samefrequency deposit energy at locations in the plasma surrounding thetarget that differ by less than the heat diffusion distance, where theseterms have a meaning following the standard practice well-known to thoseskilled in the art of ICF.

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Direct Drive with the Krypton Fluoride Laser”, 24^(th) Symposium onFusion Engineering, Chicago, Ill. 29 Jun. 2011

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What is claimed is:
 1. A laser system comprising: a plurality of pulsedlasers that emit laser pulses, the plurality of pulsed lasers configuredsuch that all of the plurality of pulsed lasers emit a laser pulse thatirradiates a target within a same time window of less than about 100 ns,at least two of the pulsed lasers having different central opticalfrequencies such that the central optical frequencies of theirrespective emitted laser pulses differ by more than 1 THz.
 2. The lasersystem of claim 1, wherein the plurality of pulsed lasers are configuredsuch that each pulsed laser emits a laser pulse that irradiates thetarget within the same time window such that the target releasesthermonuclear energy in response to the irradiation.
 3. The laser systemof claim 1, further comprising a laser controller that controls thepulsed lasers such that each pulsed laser irradiates the target with alaser pulse substantially simultaneously with the other pulsed lasers.4. The laser system of claim 1, wherein the plurality of pulsed laserscomprise at least 512 and less than 262,145 pulsed lasers.
 5. The lasersystem of claim 1, wherein the plurality of pulsed lasers are configuredto deliver the laser pulses to the target in a substantially sphericaldistribution.
 6. The laser system of claim 5, wherein the plurality ofpulsed lasers each comprises an exit aperture and the plurality of exitapertures are distributed substantially spherically around the target.7. The laser system of claim 4, wherein a distribution of the centraloptical frequencies of the pulsed lasers are correlated with a directionof propagation and a focal spot location of the respective laser pulsesemitted by the pulsed lasers toward the target according to apredetermined prescription.
 8. The laser system of claim 7, wherein thepredetermined prescription substantially maximizes a spatial uniformityof intensity of the plurality of laser pulses at a surface of the targetas computed from a ratio of the root mean square variation in theintensity over the surface of the target to the average value of theintensity over a time interval during which the plurality of laserpulses irradiate the target surface.
 9. The laser system of claim 7,wherein according to the predetermined prescription, a variation inspatial uniformity of intensity of the plurality of laser pulses at asurface of the target as computed from a ratio of the root mean squarevariation in the intensity over the surface of the target to the averagevalue of the intensity over a time interval during which the pluralityof laser pulses irradiate the target surface is less than about 0.25%.10. The laser system of claim 7, wherein a smoothing rate of thesummation of the plurality of laser pulses of the laser system at thetarget is substantially maximized at a spatial scale length of betweenabout 10 and about 100 microns, according to the predeterminedprescription.
 11. The laser system of claim 7, wherein the plurality oflaser pulses from the laser system that irradiate the target aresubstantially smoothed at a rate faster than about 30 THz, according tothe predetermined prescription.
 12. The laser system of claim 7, whereina central optical wavelength of each pulsed laser is between about 250nm and 2500 nm, and the root mean square bandwidth of the laser systemis greater than about 1 THz.
 13. The laser system of claim 7, whereintemporal pulse shapes of the at least two of the pulsed lasers havingdifferent central optical frequencies are substantially different fromeach other.
 14. The laser system of claim 7, wherein optical states ofpolarization of at least two laser pulses that irradiate the target fromdifferent respective pulsed lasers are substantially different.
 15. Thelaser system of claim 7, wherein a temporal pulse width of at least oneof the plurality of laser pulses is less than about 50 ps.
 16. The lasersystem of claim 7, wherein a first temporal pulse width of a first laserpulse of the plurality of laser pulses is between about 1 ns and 100 ns,and a second temporal pulse width of a second laser pulse of theplurality of laser pulses is less than about 50 ps.
 17. The laser systemof claim 7, wherein at least two of the plurality of laser pulsesirradiate the target surface at substantially different times.
 18. Thelaser system of claim 7, wherein an angle between propagation directionsof any two laser pulses from respective pulsed lasers whose centraloptical frequencies differ by less than about 250 THz is greater thanabout 0.01 radians.
 19. A laser system comprising: a plurality of atleast 512 and less than 262,145 pulsed lasers that emit laser pulsestoward a target, at least two of the pulsed lasers having differentcentral optical frequencies such that the central optical frequencies oftheir respective emitted laser pulses differ by more than about 1 THz; aplurality of exit apertures spatially distributed around the target suchthat the laser pulses from each of the plurality of pulsed lasers passesthrough a separate one of the plurality of exit apertures to irradiatethe target from a different direction; and a laser controller thatcontrols the plurality of pulsed lasers such that all of the pluralityof pulsed lasers irradiate the target with a laser pulse within a sametime window of less than about 100 ns.
 20. The laser system of claim 19,wherein a distribution of the central optical frequencies of the pulsedlasers are correlated with a direction of propagation and a focal spotlocation of the respective laser pulses emitted by the pulsed laserstoward the target according to a predetermined prescription.
 21. Thelaser system of claim 20, wherein the predetermined prescriptionsubstantially maximizes a spatial uniformity of intensity of theplurality of laser pulses at a surface of the target as computed from aratio of the root mean square variation in the intensity over thesurface of the target to the average value of the intensity over a timeinterval during which the plurality of laser pulses irradiate the targetsurface.
 22. The laser system of claim 20, wherein according to thepredetermined prescription, a variation in spatial uniformity ofintensity of the plurality of laser pulses at a surface of the target ascomputed from a ratio of the root mean square variation in the intensityover the surface of the target to the average value of the intensityover a time interval during which the plurality of laser pulsesirradiate the target surface is less than about 0.25%.
 23. A method ofdriving an inertial confinement fusion reaction for inertial fusionenergy generation, the method comprising: emitting a plurality of laserpulses from a plurality of pulsed lasers, the central opticalfrequencies of at least two of the pulsed lasers being different fromeach other by more than about 1 THz; and directing the plurality oflaser pulses toward a target from different exit apertures alongdifferent propagation directions such that all of the plurality of laserpulses irradiate different portions of the target within a same timewindow of less than about 100 ns.
 24. The method of claim 23, furthercomprising correlating a distribution of the central optical frequenciesof the pulsed lasers with a direction of propagation and a focal spotlocation of the respective laser pulses emitted by the pulsed laserstoward the target according to a predetermined prescription.
 25. Themethod of claim 24, wherein according to the predetermined prescription,a variation in spatial uniformity of intensity of the plurality of laserpulses at a surface of the target as computed from a ratio of the rootmean square variation in the intensity over the surface of the target tothe average value of the intensity over a time interval during which theplurality of laser pulses irradiate the target surface is less thanabout 0.25%.
 26. The method of claim 24, further comprising correlatinga distribution of optical pulse shapes of the pulsed lasers with thedistribution of propagation and focal spot location of the respectivelaser pulses emitted by the pulsed lasers toward the target according tothe predetermined prescription.